Light Rail Technical Terms#

At-Grade#

A segment of rail line that runs at the same level as the surrounding street, with trains sharing the right-of-way with vehicles, cyclists, and pedestrians at signalized crossings. The opposite of grade-separated (elevated or tunneled) alignments.

Why it matters: The 1 Line runs at-grade for 4.5 miles in the median of Martin Luther King Jr. Way South through the Rainier Valley, with 28 signalized crossings and partial gates. This cost-saving decision from the 1990s has resulted in 136 collisions between trains and vehicles, people, or objects since 2009. Sound Transit has stated that all future Link extensions will be fully grade-separated. Understanding at-grade trade-offs helps advocates evaluate alignment alternatives for planned extensions.

See also: Grade Separation, Guideway

Learn more: Sound Transit: Modes of Service | The Urbanist: Rainier Valley Safety Plan

Capital Investment Grants (New Starts)#

The Federal Transit Administration’s primary grant program for funding new fixed-guideway transit infrastructure, including new or extended light rail, commuter rail, heavy rail, bus rapid transit, and streetcar lines. Formally authorized under 49 U.S.C. § 5309, the program has three categories: New Starts (projects seeking $150 million or more in federal funds, or with a total cost of $400 million or more), Small Starts (lower-cost projects), and Core Capacity (expansions of existing lines already at or near capacity). Projects must complete a multi-year process – Project Development, then Engineering – before receiving a Full Funding Grant Agreement or other construction grant.

Why it matters: New Starts is how Sound Transit has financed a major share of Link light rail construction. Sound Transit requested a $500 million FFGA for the initial Central Link segment in 2002, covering roughly one-quarter of capital costs; the agreement was executed in October 2003. The Lynnwood Link Extension received a $1.17 billion FFGA in December 2018, part of a $3.26 billion total project budget. Planned extensions including Ballard Link and West Seattle Link depend on future New Starts awards to close their funding gaps. The program funds roughly half the capital cost of competitively selected rail and BRT projects and requires projects to demonstrate mobility improvements, environmental benefits, cost-effectiveness, and a stable local financial commitment. Projects requesting less than a 50% federal match receive higher evaluation ratings, giving agencies an incentive to maximize local funding.

Learn more: Sound Transit: Application for Lynnwood Link Federal Grant | Sound Transit: Initial Federal Funding Request for Central Link

Communications-Based Train Control (CBTC)#

A modern railway signaling system defined by IEEE Standard 1474 that uses continuous, bidirectional radio communication between trains and wayside equipment to determine train positions in real time — independent of traditional track circuits. Unlike fixed-block signaling, where tracks are divided into predetermined sections that only one train may occupy, CBTC enables “moving block” operation: each train continuously reports its exact location, speed, and braking distance, allowing the system to dynamically calculate the minimum safe following distance between trains.

Why it matters: Sound Transit’s Link light rail does not use CBTC. The system instead uses automatic block signaling with Automatic Train Protection and train-to-wayside communication loops — a more traditional approach partly necessitated by the 1 Line’s at-grade segments in the Rainier Valley, where trains must integrate with the traffic signal network at surface crossings. CBTC systems can achieve headways as short as 60–90 seconds, compared to the four-minute peak headways Sound Transit targets for combined 1 Line/2 Line service. CBTC is also a prerequisite for fully automated (driverless) operation, which some advocates have proposed as an alternative for future Link extensions like Ballard and West Seattle. Understanding CBTC helps advocates evaluate whether signaling upgrades in the existing Downtown Seattle Transit Tunnel could increase capacity enough to defer or eliminate the need for a second downtown tunnel.

Learn more: IEEE 1474.1-2025: CBTC Performance and Functional Requirements | The Urbanist: Driverless Metro as Alternative for Ballard and West Seattle

Cathodic Protection#

An electrochemical corrosion-prevention system that collects stray electrical current and grounds it into a sacrificial metal anode, protecting structural steel from corrosion. Used on electrified rail systems to prevent damage to metal infrastructure near tracks.

Why it matters: Cathodic protection is critical to the 2 Line’s crossing of the I-90 Homer M. Hadley floating bridge. Stray current from the 1,500V DC overhead power system could corrode the bridge’s steel anchor cables, which hold the floating spans in place. Sound Transit installed a cathodic protection system with collection mats, ground electrodes, and a stray current monitoring system. Engineers also designed lightweight concrete track-mounting blocks and corkelast shock-absorbing material that resist stray current leakage. The system’s performance was a key milestone for the March 2026 cross-lake opening.

Learn more: Sound Transit: Bridging the Gap on the I-90 Floating Bridge | Sound Transit: Crosslake Update

Crossover#

A pair of switches (turnouts) connecting two parallel tracks, allowing a train to move from one track to the other. Crossovers enable trains to reverse direction, bypass obstructions, or switch between tracks without reaching a terminal station.

Why it matters: The Downtown Seattle Transit Tunnel (DSTT) was originally built without crossovers in most of its middle sections, which Sound Transit has identified as a critical operational vulnerability. Without mid-tunnel crossovers, a single disabled train can shut down the entire tunnel because trains cannot single-track around the obstruction. Adding a crossover in the DSTT is part of planned resiliency improvements. For the West Seattle Link Extension, relocating the crossover from the south side of Alaska Junction station to the north side is projected to save $190-235 million in construction and right-of-way costs.

See also: Interlocking, Pocket Track, Guideway

Learn more: The Urbanist: Sound Transit Building Blocks | The Urbanist: Sound Transit Unveils Cost Savings Concepts for West Seattle and Ballard Light Rail

Cut-and-Cover Tunnel#

A tunnel built from the surface down: a trench is excavated along the tunnel route, the tunnel structure is constructed within the trench, a roof is placed over it, and the surface is restored. Faster and cheaper than bored tunnels for shallow depths (under ~40 feet) but causes significant surface disruption during construction.

Why it matters: Cut-and-cover is one of three tunneling methods Sound Transit uses, alongside TBM-bored tunnels and sequential excavation method (SEM) tunnels. The original Downtown Seattle Transit Tunnel (1987-1990) used cut-and-cover for station excavations and TBM boring for the running tunnels between stations. For the planned Ballard Link Extension downtown tunnel, three stations – Denny, South Lake Union, and Seattle Center – are planned as cut-and-cover construction, while Midtown and Westlake would use SEM at greater depth. Cut-and-cover stations cost significantly less than SEM stations but require temporarily closing or rerouting surface streets. The choice of tunnel method for each station is driven by depth, geology, underground utility density, and surface impact tolerance.

Learn more: Sound Transit: New Downtown Seattle Light Rail Tunnel | Sound Transit: Ballard Link Extension

Emergency Walkway#

A raised pathway alongside tracks within tunnels and on elevated guideways that provides a safe evacuation route for passengers during emergencies. Required by NFPA 130 to be a minimum of 44 inches wide.

Why it matters: Link’s tunnel segments include emergency walkways, cross passages connecting twin-bore tunnels, and ventilation shafts – all designed for passenger evacuation. The University Link tunnel has 16 cross passages between its twin bores, spaced at regular intervals per NFPA 130 requirements (no more than 800 feet apart). Sound Transit’s official evacuation instructions tell passengers to stay on the train and wait for instructions rather than entering the trackway, because of electrified equipment and switch hazards. On elevated segments, passengers exit onto an adjacent pathway and wait for emergency personnel.

Learn more: Sound Transit: Link Light Rail Evacuation Instructions | Sound Transit: Spring Update on Link Construction

Fire-Life Safety (FLS)#

The integrated set of systems and design features in transit tunnels and stations that protect passengers and staff from fire-related hazards. Includes emergency ventilation systems, fire detection and suppression, emergency lighting, communication systems, evacuation walkways, cross passages, and fire-rated construction materials. Governed by NFPA 130.

Why it matters: Fire-life safety systems in Link’s tunnels directly constrain operations. Emergency ventilation fans must maintain breathable air and keep temperatures below 120 degrees F in egress paths for at least one hour during an emergency. Fan failures have caused tunnel closures: a ventilation fan malfunction in August 2025 and a fan power supply failure in September 2025 both shut down service between Westlake and UW to comply with fire codes. Sound Transit has been upgrading aging Emergency Ventilation System switches across the network – 12 of 15 stations have been upgraded. FLS requirements also affect tunnel design costs: ventilation shafts, cross passages, and emergency egress add significantly to tunnel construction budgets.

Learn more: Sound Transit: Overnight Maintenance | Sound Transit: Evacuation Instructions

Fare Gates#

Physical barriers at station entrances that require passengers to tap a valid fare (ORCA card, contactless payment, or ticket) to open before entering or exiting the paid area. Fare gates enforce payment at the point of entry rather than relying on random inspections after boarding.

Why it matters: Link light rail currently operates without fare gates, using a proof-of-payment model with random inspections by fare ambassadors. Fare compliance on Link dropped from 85% in 2019 to 61% in 2024, representing approximately ~$15 million in unrealized annual revenue. In December 2025, the Sound Transit Board advanced a motion examining where fare gates would make sense, with a potential pilot program before the end of 2026. The agency is studying BART (Bay Area Rapid Transit) as a model. Fare gate retrofits face significant challenges on Link: many stations have open, at-grade platforms without enclosed mezzanines, making it difficult to create a controlled paid area. Underground stations in the Downtown Seattle Transit Tunnel are more amenable to gates. The debate weighs capital and maintenance costs against fare revenue recovery, accessibility concerns, and rider experience.

Learn more: The Urbanist: Sound Transit Board Closes In on Fare Gate Proposal | Sound Transit: Fare Ambassadors

Full Funding Grant Agreement (FFGA)#

A contractual agreement between the Federal Transit Administration and a transit agency that commits the federal government to provide a specified maximum amount of Capital Investment Grant (New Starts) funding for a fixed-guideway transit project over multiple federal fiscal years. The FFGA defines the project scope, schedule, and total federal contribution; any cost increases after signing must be covered by the local project sponsor. Before an FFGA can be signed, FTA must complete its review, the U.S. Department of Transportation must concur, and Congress must receive 30 days’ notice.

Why it matters: An FFGA is the formal federal commitment that allows a transit agency to proceed to construction with confidence that federal funding will be available over the multi-year build period. Sound Transit has executed FFGAs for each major Link segment: the initial Central Link segment received a $500 million FFGA in October 2003 (covering roughly one-quarter of capital costs); the Lynnwood Link Extension received a $1.174 billion FFGA in December 2018 (part of a $3.26 billion total project, also supported by a $658 million federal loan). Future extensions to Ballard and West Seattle will require new FFGAs. Because FFGAs cap the federal contribution at a fixed dollar amount, cost overruns – which have affected Sound Transit’s ST3 program – are borne entirely by local revenues, primarily property tax, sales tax, and MVET.

Learn more: Sound Transit: Lynnwood Link Federal Grant Application | Sound Transit: Lynnwood Link $100 Million Federal Appropriation

Grade Separation#

A design approach where rail tracks are physically separated from road traffic by running in tunnels, on elevated guideways, or in trenches. Grade-separated lines eliminate conflicts with vehicles and pedestrians at intersections.

Why it matters: Grade separation is the most debated infrastructure decision in Sound Transit expansion planning. The 1 Line’s at-grade segment in the Rainier Valley has had ongoing safety issues, leading Sound Transit to commit to full grade separation for all future extensions. The Ballard Link Extension debate centers on whether to build an elevated/tunneled alignment (faster but far more expensive at $10.8 billion) or at-grade alternatives (cheaper but slower). Grade separation affects train speed, reliability, safety, construction cost, and neighborhood impacts.

See also: At-Grade, Guideway, Tunnel Boring Machine

Learn more: Sound Transit: West Seattle and Ballard Link Extensions | The Urbanist: Rainier Valley Safety Plan

Guideway#

The dedicated path on which a light rail train operates, including the track structure, rail, ties (or direct-fixation slab), and supporting infrastructure. Guideways can be at-grade (surface), elevated (on columns or viaducts), or underground (in tunnels).

Why it matters: Guideway type determines a light rail project’s cost, speed, reliability, and neighborhood impact. Link light rail uses all three types: tunnels through Beacon Hill, Capitol Hill, the U District, and Northgate; elevated segments through SODO, Tukwila, the Eastside, and Lynnwood; and at-grade running in the Rainier Valley. Elevated guideways are generally the most cost-effective form of grade separation, while tunnels are the most expensive but have the smallest surface footprint. Understanding guideway types helps advocates evaluate alignment alternatives.

See also: At-Grade, Grade Separation

Learn more: Sound Transit: Looking Back at Building the 1 Line to Federal Way | Sound Transit: System Expansion

Interlocking#

A section of track where switches (moveable rail segments) and signals are interconnected so that a signal can only clear a route that has been confirmed safe. Interlockings control train movements at junctions, crossovers, and locations where trains switch between tracks.

Why it matters: Interlockings are the chokepoints of a rail network. The number and placement of interlockings determine how flexibly trains can be routed, how quickly the system can recover from disruptions, and the minimum achievable headway. On Link light rail, the International District/Chinatown station area is a critical interlocking where the 1 Line and 2 Line merge and diverge. The capacity of this junction directly affects whether Sound Transit can reliably deliver 4-minute combined headways. Advocates pushing for better service frequency should understand that interlocking capacity, not just track or vehicles, often limits frequency.

See also: Headway, Grade Separation

Learn more: Sound Transit: A State of Better Repair | Sound Transit: 4-Minute Headway Testing

Low-Floor Vehicle#

A rail vehicle designed with a floor height close to platform level (typically 14-15 inches above the rail), allowing level boarding without steps. Low-floor vehicles have at least some sections where the floor is not raised over the wheel trucks (bogies).

Why it matters: Both of Sound Transit’s light rail vehicle types – the Series 1 (Kinkisharyo) and Series 2 (Siemens S700) – are 70% low-floor, meaning most of the car interior is at platform level for step-free boarding. This is an accessibility requirement under the Americans with Disabilities Act (ADA) and also speeds up boarding for all riders, including those with strollers, luggage, or mobility devices. Low-floor design is a key difference between modern light rail and older heavy rail or commuter rail systems that require high platforms or stairs.

See also: Standard Gauge, Overhead Contact System

Learn more: Sound Transit: New Link Light Rail Trains Rolling into Service | The Urbanist: Sound Transit Speeds Up Delivery of 10 Link Cars

NFPA 130#

The National Fire Protection Association’s Standard for Fixed Guideway Transit and Passenger Rail Systems. NFPA 130 specifies fire protection and life safety requirements for underground, surface, and elevated transit systems, covering emergency ventilation, evacuation walkways, cross passages, fire detection, suppression, and construction materials.

Why it matters: NFPA 130 is the governing standard for fire-life safety design in all of Link light rail’s tunnels and stations. It requires emergency walkways at least 44 inches wide, cross passages spaced no more than 800 feet apart in tunnels, and ventilation systems capable of maintaining breathable air below 120 degrees F for at least one hour during an emergency. Compliance with NFPA 130 is verified during safety certification before any new segment enters revenue service. NFPA 130 requirements also directly affect construction costs – ventilation shafts, cross passages, and emergency egress are among the most expensive elements of tunnel construction. When ventilation systems fail to meet NFPA 130 standards, Sound Transit must suspend tunnel service until the systems are restored.

Learn more: NFPA 130: Standard for Fixed Guideway Transit and Passenger Rail Systems | Sound Transit: Evacuation Instructions

Operations and Maintenance Facility (OMF)#

A large rail yard where light rail vehicles are stored overnight, cleaned, inspected, and repaired. OMFs include vehicle storage tracks, maintenance bays, wash facilities, parts warehouses, and operations control rooms.

Why it matters: OMF capacity determines how many trains can operate on the system. Sound Transit currently operates OMF Central (SODO, 104 vehicles) and OMF East (Bellevue Spring District, 96 vehicles, opened 2024). OMF South (Federal Way, 144 vehicles) is in the design phase targeting a 2031 opening, and OMF North is in early planning. Before OMF East opened, the 1 Line’s service frequency was constrained by OMF Central’s limited capacity. OMF East also includes a transit-oriented development component with planned housing and retail on 6.9 acres. Understanding OMF capacity helps advocates assess whether fleet expansion and frequency improvements are feasible.

See also: Headway, Low-Floor Vehicle

Learn more: Sound Transit: OMF South | DBIA: OMF East

Overhead Contact System (OCS)#

The network of wires suspended above rail tracks that delivers electrical power to trains. The system consists of a messenger (catenary) wire that supports a lower contact wire, connected by vertical droppers. A spring-loaded pantograph on the train’s roof presses against the contact wire to collect current.

Why it matters: Link light rail runs on 1,500V DC power delivered through the OCS – double the voltage used by most North American light rail systems. This higher voltage allows traction power substations to be spaced farther apart (roughly one mile), reducing infrastructure costs. OCS reliability directly affects service: incorrect wire tension, electrical arcing, and weather-related sagging have caused Link service disruptions. In 2025, Sound Transit replaced 400 feet of contact wire at the University of Washington station to restore full train speeds. The OCS wire zigzags slightly across the track to wear down the pantograph’s carbon contact shoe evenly.

Learn more: Sound Transit: What We’re Doing to Make Link Service More Reliable | Sound Transit: A State of Better Repair

Pantograph#

A spring-loaded, articulated device mounted on the roof of a light rail vehicle that presses upward against the overhead contact wire to collect electrical current. The top of the pantograph has a carbon contact shoe (or strip) that wears down gradually from friction with the wire and must be periodically replaced.

Why it matters: The pantograph is the physical link between the train and its power source. Sound Transit describes pantographs as the “antennae on top of light rail vehicles” that connect to the overhead contact system. Pantograph-wire contact quality affects acceleration, braking, and energy efficiency. Poor contact causes electrical arcing, which can damage both the wire and the pantograph, leading to service delays. The OCS contact wire zigzags slightly so the carbon shoe wears evenly across its width rather than cutting a single groove.

Learn more: Sound Transit: What We’re Doing to Make Link Service More Reliable | Sound Transit: A State of Better Repair

Platform Screen Doors#

Sliding glass or metal barriers installed at the edge of train station platforms that align with train doors and open in synchronization with them, separating the waiting area from the tracks. Full-height versions run floor-to-ceiling; half-height versions (platform edge doors) are waist-high barriers that prevent falls and track intrusions.

Why it matters: Link light rail does not have platform screen doors, but they are a recurring topic in advocacy discussions about system capacity and safety. PSDs require precise train stopping (typically within centimeters), which depends on advanced signaling like Communications-Based Train Control (CBTC) – a system Link does not currently use. Sound Transit has installed between-car safety barriers (bollards) at 12 stations to prevent visually impaired riders from mistaking the gap between cars for a door, but these are not PSDs. Variable consist lengths (Link runs 2-, 3-, and 4-car trains) further complicate PSD implementation since doors must align precisely. PSDs have been discussed in the context of potential DSTT upgrades and the possibility of automated operations on future grade-separated extensions. No transit system in the Pacific Northwest currently has PSDs, though Vancouver’s TransLink is studying feasibility for SkyTrain.

See also: Train Control System, Low-Floor Vehicle

Learn more: The Urbanist: Sound Transit Deploys Light Rail Safety Barriers | The Urbanist: Sound Transit Building Blocks

Pocket Track#

A short section of track placed between two mainline tracks, connected at both ends via crossovers. Allows a train to pull off the mainline, reverse direction, and reenter going the opposite way – enabling short-turn service or emergency turnarounds without reaching a terminal station.

Why it matters: Link has a pocket track at International District/Chinatown station, which is operationally significant as the junction where the 1 Line and 2 Line converge. During 2 Line testing in late 2025, out-of-service 2 Line trains used this pocket track to interline between regular 1 Line trains, practicing the service pattern for full interlined operations. Pocket tracks give operators flexibility to short-turn trains during disruptions rather than running them the full length of a line, which is critical for maintaining headways when problems occur on one end of the system.

See also: Crossover, Interlocking, Tail Track

Learn more: The Urbanist: 2 Line Testing Enters a New Phase | The Urbanist: Sound Transit Building Blocks

Regenerative Braking#

An energy recovery system where a train’s electric traction motors operate as generators during braking, converting kinetic energy back into electrical energy. The recovered power is fed back into the overhead contact system for use by other trains accelerating on the same electrical section.

Why it matters: Both of Link’s vehicle types use regenerative braking. The Series 2 (Siemens S700) vehicles provide full dynamic braking from 55 mph down to 0.5 mph, with friction brakes engaging only for the final stop. The Series 1 (Kinkisharyo) vehicles use IGBT-VVVF inverter-controlled propulsion with regenerative capability. The recovered energy returns to Link’s 1,500V DC overhead catenary, where it can power other trains. Since December 2020, Link runs on 100% renewable electricity from wind and hydropower, making regenerative braking part of a fully renewable traction energy cycle. Regenerative braking also reduces wear on mechanical brake components, lowering maintenance costs.

Learn more: Siemens Mobility: Seattle Light Rail Vehicles | Kinkisharyo: Sound Transit Project

Revenue Service#

The operation of transit vehicles carrying fare-paying passengers on a scheduled route, as distinct from testing runs, simulated service, or non-revenue maintenance trips. A new rail segment does not enter revenue service until it has completed required safety certifications and operational testing.

Why it matters: “Revenue service” is the milestone that marks when a new light rail segment is actually open to the public. Before revenue service, new Link extensions go through several testing phases: initial system integration testing, pre-revenue testing (training operators and verifying that vehicles, stations, and utilities work together), and simulated service (running trains on the full schedule without passengers to validate operations). Sound Transit and WSDOT’s State Safety Oversight program must certify the segment safe before revenue service can begin. For the Lynnwood Link Extension, pre-revenue testing began in June 2024, simulated service ran in July–August 2024, and revenue service started August 30, 2024. The 2 Line opened to partial revenue service April 27, 2024 (South Bellevue to Redmond Technology), with the cross-lake segment to downtown Seattle scheduled for March 28, 2026. Advocates tracking expansion timelines should understand that “construction complete” and “revenue service” are separated by months of testing.

Learn more: Sound Transit: Pre-Revenue Testing Underway on Lynnwood Link Extension | Sound Transit: East Link Extension

Retained Cut#

A below-grade alignment where tracks run in an open or covered trench with retaining walls on either side, but without a full tunnel enclosure. When a lid is placed over the trench, it is called a “lidded retained cut,” which restores surface use above the tracks. Less expensive than tunneling but requires more surface footprint than a bored tunnel.

Why it matters: Link uses retained cuts on several segments. The Lynnwood Link Extension runs approximately 4.4 miles of its 8.5-mile length in retained cut or fill guideway parallel to I-5. The West Seattle Link Extension plans Avalon station as a lidded retained cut south of SW Genesee Street beneath 35th Avenue SW. On the 2 Line, the Spring District station in Bellevue is a trenched (retained cut) configuration. Retained cuts are a middle option between at-grade and tunneled alignments: they provide grade separation from road traffic at lower cost than tunnels but require acquiring a wider right-of-way than bored tunnels and can create barriers in neighborhoods if not lidded.

Learn more: FTA: Lynnwood Link Extension Project Profile | Sound Transit: West Seattle Link Extension

Safety Certification#

The process by which a new rail transit segment or system is formally verified as safe for passenger operations before revenue service begins. In Washington state, safety certification is overseen by the WSDOT Public Transportation Division, which runs the state’s federally certified Rail Safety Oversight (SSO) program under 49 U.S.C. § 5329.

Why it matters: WSDOT’s SSO program – a partnership with the Federal Transit Administration – oversees safety for Sound Transit’s Link Light Rail, the T Line (First Hill Streetcar), and other rail transit in Washington. Before a new Link segment can open, Sound Transit must complete a multi-step safety and security certification process: construction conformance audits, FTA-required safety and security exercises with emergency responders, and resolution of all outstanding corrective actions. WSDOT’s SSO team conducts risk-based inspections, reviews safety plans, and can require corrective action before approving revenue service. For the Lynnwood Link Extension, a safety certification audit was completed in August 2024 (approximately one month before the August 30, 2024 opening). Understanding safety certification helps advocates distinguish between construction delays and the required testing and approval phases, and explains why a line that appears physically complete still cannot carry passengers for several months.

See also: Revenue Service, At-Grade, Grade Separation

Learn more: WSDOT: Rail Safety Oversight Program Standard | Sound Transit: Pre-Revenue Testing Underway on Lynnwood Link Extension

Sequential Excavation Method (SEM)#

A tunnel construction technique that excavates soil in small, controlled sections using conventional equipment (excavators and roadheaders) rather than a tunnel boring machine. After each section is excavated, crews immediately spray pressurized concrete (shotcrete) onto exposed surfaces and install steel lattice girders for structural support. Also called the New Austrian Tunneling Method (NATM).

Why it matters: Sound Transit has used SEM for several Link segments where tunnels are too short to justify mobilizing a TBM or where complex station geometries are needed. The downtown Bellevue tunnel on the 2 Line used SEM to excavate a 1,985-foot, 34-foot-wide tunnel under 110th Avenue NE, completed five months ahead of schedule in 2018. Beacon Hill station’s platform and concourse tunnels – at 45 feet wide, the largest soft-ground SEM tunnels in the United States at the time – were also built with SEM. For the planned Ballard Link Extension downtown tunnel, Midtown and Westlake stations would use SEM at greater depth, while shallower stations would use cut-and-cover. SEM’s advantage is minimizing surface disruption – no long-term street closures or open trenches – while allowing crews to adapt in real time to changing ground conditions.

Learn more: Sound Transit: East Link Tunnel Excavation Nears Completion | Sound Transit: Bellevue Light Rail Tunnel Progress Update

Standard Gauge#

The distance between the inner edges of the two rails on a track, set at 4 feet 8.5 inches (1,435 mm). Standard gauge is used by approximately 60% of the world’s rail lines, including Link light rail, Amtrak, freight railroads, and most North American transit systems.

Why it matters: Link light rail uses standard gauge, the same as freight railroads and Sounder commuter rail. This is a common choice for North American light rail systems and means the track geometry follows well-established engineering standards. Standard gauge matters when discussing shared corridors, vehicle procurement (most manufacturers build for standard gauge), and connections between different rail systems.

See also: Guideway

Learn more: Sound Transit: Modes of Service | FTA: National Transit Database Glossary

Stray Current#

Electrical current that leaks from a rail system’s intended circuit and flows through unintended paths such as the ground, structural steel, or water. On electrified rail systems, stray current can corrode metal infrastructure including bridge cables, rebar, and utility pipes.

Why it matters: Stray current is a concern on all electrified rail systems, but it received heightened scrutiny for the 2 Line’s I-90 floating bridge crossing. Because stray current could corrode the steel anchor cables that hold the floating bridge in place, Sound Transit installed a cathodic protection system, stray current collection mats, and a monitoring system. Link’s 1,500V DC power and ungrounded running rails are designed to minimize leakage. Sound Transit notes that traction power substations are designed to “cut power” when excess stray current is detected, which can cause service disruptions.

Learn more: Sound Transit: Bridging the Gap on the I-90 Floating Bridge | Sound Transit: What We’re Doing to Make Link Service More Reliable

Tail Track#

A section of track extending beyond the last station platform at a terminal station. Allows trains to pull forward past the platform before reversing direction, speeding up turnaround operations and providing storage space for out-of-service vehicles.

Why it matters: Tail tracks affect how quickly trains can turn around at the end of a line and thus how efficiently a given fleet of trains can maintain headways. Link has tail tracks at Federal Way (1 Line southern terminus) and Downtown Redmond (2 Line eastern terminus). For the planned West Seattle Link Extension, Sound Transit has proposed eliminating the tail tracks south of Alaska Junction station as a cost-saving measure ($190-235 million savings), arguing they would have no operational effect given the planned crossover configuration. However, the lack of tail tracks at other Link stations (Stadium, Northgate, Lynnwood) has been identified as a hindrance to short-turn operations. Whether tail tracks are necessary depends on the turnaround time needed at each terminal relative to the planned headway.

See also: Pocket Track, Crossover, Headway

Learn more: West Seattle Blog: Sound Transit Cost-Cutting Possibilities | The Urbanist: Sound Transit Outlines Everett Link Cost Reductions

Traction Power Substation (TPSS)#

A facility that converts commercial alternating current (AC) electricity from the utility grid into the direct current (DC) voltage that powers light rail trains through the overhead contact system. TPSS units are located along the rail corridor at regular intervals.

Why it matters: Link light rail operates on 1,500V DC power, double the 750V standard used by most North American light rail systems. This higher voltage allows Sound Transit to space substations approximately one mile apart instead of every half-mile, reducing the number of substations needed and lowering infrastructure costs. Siemens Mobility has supplied TPSS units for Link extensions since the University Link project, including 13 substations for the Northgate and East Link lines. TPSS failures are the least common type of Link service disruption, but when they occur, they cause an average of six hours of delay. Since December 2020, Link light rail has run on 100% renewable electricity from wind and hydropower.

Learn more: Sound Transit: What We’re Doing to Make Link Service More Reliable | Mass Transit: Siemens Traction Power Technology Chosen for Seattle’s Light Rail Expansion

Train Control System#

The signaling and safety systems that manage train movements, enforce speed limits, and maintain safe spacing between trains. Link uses automatic block signaling with Automatic Train Protection (ATP) and train-to-wayside communication loops rather than the more advanced Communications-Based Train Control (CBTC) used by some metro systems.

Why it matters: Link’s train control system automatically brakes trains that exceed 58 mph on grade-separated sections or 38 mph on surface sections. The system uses Centralized Traffic Control with computer-managed turnouts. The original Central Link installation included 87 signals, 228 train-to-wayside communication cabinets, and 52 switch machines, with additional equipment added as the system has expanded. Sound Transit completed a major advanced signaling installation in the Downtown Seattle Transit Tunnel in late 2025 and early 2026, requiring multiple weekend closures with bus bridges between Capitol Hill and Stadium. Link does not use CBTC partly because the 1 Line’s 4.5 miles of at-grade running in the Rainier Valley – with 28 signalized crossings – complicates the continuous radio communication CBTC requires. CBTC enables shorter headways through “moving block” signaling (trains tracked in real time rather than by fixed track sections) and is a prerequisite for platform screen doors and automated operations.

See also: Interlocking, Platform Screen Doors, At-Grade

Learn more: Sound Transit: Upcoming Planned Closures on the 1 Line | Sound Transit: Overnight Maintenance

Tunnel Boring Machine (TBM)#

A large cylindrical machine that excavates tunnels by rotating a cutterhead against the rock or soil face while simultaneously installing precast concrete lining segments behind it. TBMs used for Link light rail tunnels have 21-21.5-foot-diameter cutterheads and weigh 600+ tons with trailing gear.

Why it matters: TBMs have built approximately 14 miles of twin-bore tunnel for Link light rail. The Beacon Hill tunnels (2006-2008) used a 360-ton Mitsubishi TBM nicknamed the “Emerald Mole” that completed two 4,300-foot tunnels, emerging within 5 millimeters of its target. The University Link tunnels (2009-2012) used three TBMs that bored 3.15 miles through challenging geology 300 feet below Volunteer Park. The Northgate Link tunnels (2014-2016) used two 600-ton TBMs that mined 3.5 miles beneath the Ship Canal. A second downtown transit tunnel planned for the Ballard Link Extension will require future TBM work. TBM-bored tunnels are the most expensive but most reliable type of guideway.

See also: Grade Separation, Guideway

Learn more: Sound Transit: 8 Facts About Northgate Link Tunneling | Sound Transit: 9 Quick Facts About How U-Link Was Built

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