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SEA Underground
Interior of a Innovia APM 100 car used on the SEA Underground, refurbished with color displays
Overview
OwnerPort of Seattle
LocaleSeattle–Tacoma International Airport
Stations6
Service
TypePeople mover
Services3
Rolling stock21 Innovia APM 100 vehicles
Ridership16 million (2015)[1]
History
OpenedJuly 1973 (1973-07)
Technical
Line length1.67 mi (2.69 km)
Number of tracks1
CharacterUnderground
Operating speed27 mph (43 km/h)
Route map
Legend
North Satellite (N gates)
Concourse C (C gates)
Main Terminal North (D gates)
 SeaTac/Airport station Link light rail
Main Terminal South (A gates)
Concourse B (B gates)
South Satellite (S gates)
Handicapped/disabled access all stations accessible
This diagram:

The SEA Underground, formerly called the Satellite Transit System (STS), is an automated people mover (APM) system operating in the Seattle–Tacoma International Airport in SeaTac, Washington, United States. Originally opening in 1973, the SEA Underground is one of the oldest airport people mover systems in the world. It was designed to quickly transport passengers to and from the North and South Satellites, and around the airport's Main Terminal.

History

[edit]

The system was approved for construction in 1969, to be built alongside the new satellite terminals as the first inter-terminal train system in the United States.[2] It was completed in 1972 at a cost of $5 million and opened to the public in July 1973.[3][4] That makes it the second oldest airport people mover system in the United States[5] (after Tampa International Airport). The opening was delayed due to a dispute between the Port of Seattle and Westinghouse, the manufacturer of the system, over contracted costs.[6]

The system opened in 1973 at a total cost of $14 million. The original system consisted of nine vehicles;[7] an additional three were added in the mid-1970s. The system was designed to have a capacity of 14,400 passengers per hour and travel at a maximum speed of 27 miles per hour (43 km/h).[8]

The original C-100 vehicles were built by Westinghouse and had a maximum capacity of 102 passengers.[9] The average travel time for the two loops was 3.3 minutes, and 1.8 minutes on the shuttle, and each vehicle was estimated to amass 47,000 miles (76,000 km) annually.[9]

In 1999, the Port of Seattle authorized $142 million to completely overhaul the entire SEA Underground system.[10] The overhaul included all aspects of the system including trains, controls, power supplies, stations, emergency ventilation systems and maintenance shops.[10] The upgrade and modernization was completed in 2003. The line now uses Innovia APM 100 vehicles (a modernized version of the original C-100 vehicles) with the Cityflo 650 CBTC (communications-based train control) system for its automated operation.[11]

The Port of Seattle renamed the system to SEA Underground in March 2022 and assigned new names to the lines using colors: Green Line, Blue Line, and Yellow Line. Those colors were also added on the walls next to and behind the station doors.[12]

Layout and operation

[edit]
Main Terminal South station, showing the color-coded Blue Line boarding area on the left and the Yellow Line on the right

SEA Underground is located within secure areas of the airport. The system consists of six stations serving each of the four gate concourses extending from the main terminal (Concourses A, B, C and D), and the North and South Satellite terminals. Each station is equipped with platform edge doors. The system consists of two loops serving the satellite terminals and a third line connecting the two loops in the main terminal.[9][13]

  • The Green Line (North Train Loop) is 4,100 feet (1,200 m)[9] in length and has stations in the north end of the Main Terminal (near Concourse D), Concourse C and the North Satellite (N gates).[13]
  • The Blue Line (South Train Loop) is 3,700 feet (1,100 m)[9] in length and has stations in the south end of the Main Terminal (near Concourse A), Concourse B and the South Satellite (S gates).[13]
  • The Yellow Line (Shuttle Train between A and D Stations) is 1,000 feet (300 m)[9] in length and has stations at both the north end of the Main Terminal (near the D Concourse) and the south end of the Main Terminal (near the A Concourse), and serves as a connection between the North and South Terminal Transit Loops.[13]

The system has 21 APM vehicles, with about 13 in service at any given time: two 3-car trains on the Green and Blue Lines and a single car on the Yellow Line.[1] Approximately 40 percent of Sea-Tac Airport passengers use the SEA Underground system. Maintenance is generally conducted on weekends after cars are moved into one of two shops via a lateral transfer table.[14]

The system has a 99.9% uptime rate. When track maintenance is required on the Green and Blue Lines, the work is typically done overnight, and a single train operates in a shuttle mode. If the system experiences a loss of traction power, cars have enough power storage to allow them to reach the next station. While most APM systems are operated by the system builder, the Port of Seattle is one of the few owners that also operate and maintain their APM system. The Port estimates that this arrangement results in a 40% savings in maintenance costs.[1]

Public art

[edit]

As part of its 2003 renovation, public art projects were included in the scope of the project. The main terminal's south station features a series of 56-plus flowers cast of aluminum and aluminum/resin mix created by Nancy Blum.[15] In the main terminal's north station is a series of nine paintings created by Karen Ganz representing various travelers.[16]

See also

[edit]

References

[edit]
[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

SEA Underground

View on Grokipedia
from Grokipedia
The SEA Underground is an automated (APM) system at (SEA) in , Washington, designed to transport passengers efficiently between the main terminal and the airport's satellite concourses A, B, C, D, N, and S. Opened in 1973 as part of a major airport expansion, it is the second oldest airport in the United States and was originally known as the Satellite Transit System (STS). The system operates entirely underground within a fixed-guideway network, utilizing Westinghouse GRT (group rapid transit) technology with automated, driverless trains that run continuously without requiring TSA re-screening for connecting flights. The SEA Underground consists of three distinct lines to cover the airport's layout: the Blue Line (South Train Loop), which serves gates in Concourses A, B, and S; the Green Line (North Train Loop), connecting gates in Concourses C, D, and N; and the Yellow Line (Terminal Shuttle), providing direct service between Concourses A and D. Each loop measures approximately 1.2 kilometers (0.75 miles), with the shuttle spanning 300 meters (0.19 miles), allowing passengers to reach any gate in 2 to 5 minutes at headways of 1 to 3 minutes. Trains are composed of two-car units, each accommodating up to 40 passengers, with a design capacity exceeding 9,000 passengers per hour per loop; by the early , annual ridership had reached about 20 million. Stations are located post-security at key points, such as near Gate A3, Gates B20/21, below S Gates, near Gate D2, Gate C11, and below the N Concourse, with digital signage available in eight languages including English, Spanish, Chinese, and Korean. Renamed to SEA Underground in 2025 to better reflect its role in the airport's modern infrastructure, the system has undergone upgrades including modernizations and station renovations, such as the addition of the glass art installation "Cascadia" at the C Gates station. It primarily serves major airlines like (at C, D, N gates), Delta and United (at A, S gates), and international carriers (at A gates), contributing to SEA's status as a key hub handling over 50 million passengers annually. Operating costs have historically been low, at approximately $0.07 to $0.09 per passenger trip, underscoring its efficiency in supporting the airport's growth from a regional facility in the to a major international gateway.

History

Planning and Construction

In 1969, the approved the development of the Satellite Transit System (STS) as part of a broader expansion to accommodate increasing air traffic, aiming to connect the main terminal building to new satellite concourses via an automated underground transit network. This approval followed the finalization of an expansion plan in 1968, which included the addition of north and south satellite terminals accessed by driverless shuttle trains in subway loops. On June 29, 1969, the Port signed a contract with for the system's core components, marking a key milestone in the planning phase. The design specifications for the STS emphasized efficiency and reliability, selecting automated people mover (APM) technology to transport passengers swiftly between facilities. The system featured a total guideway length of approximately 1.67 miles, comprising two loops—3,700 feet for the south loop and 4,100 feet for the north loop—plus a 1,000-foot shuttle segment, all operating underground beyond security checkpoints. Westinghouse's Shuttle-Loop Transit (SLT) technology was chosen, utilizing rubber-tired, electrically powered vehicles guided by an I-shaped beam, with each vehicle designed to carry up to 106 passengers at speeds of up to 27 mph. The estimated capital cost for the STS was $14 million in 1973 dollars, including $5.3 million for hardware and the remainder for tunnels, stations, and installation, making it one of the earliest major implementations of at an airport. Construction began in late as part of the overall $175 million expansion program, which had escalated from an initial $44 million proposal due to and scope changes. for the broader occurred in October 1968, but STS-specific work, including tunnel excavation in a developed environment, followed the 1969 contract to integrate with the ongoing satellite terminal builds. Key engineering efforts focused on creating the underground fixed-guideway using group principles, with construction completed by mid-1973 to align with the opening of the North and South Satellites. The incorporated Westinghouse's systems from the outset, initially deploying two one-car per loop to ensure operational readiness upon integration.

Opening and Early Operations

The SEA Underground, an automated system at Seattle-Tacoma International Airport, officially opened in July 1973 as part of the airport's major expansion to accommodate growing air traffic. The initial setup featured nine vehicles operating on two separate loops—the North Loop connecting the main terminal to Concourse A and the North Satellite, and the South Loop linking to Concourse C and the South Satellite—along with a shuttle service between Concourses B and D in the central terminal. This underground configuration allowed secure, post-security passenger transport between the main terminal and satellite facilities, reducing walking distances and enhancing efficiency for travelers. The system, supplied by Westinghouse, was one of the earliest airport people movers in the United States, designed with a capacity of up to 14,400 passengers per hour. Early operations emphasized reliability and speed, with vehicles reaching a maximum of 27 (43 km/h) along the 1.67-mile (2.69 km) guideway, completing a full loop in approximately five minutes. The automated controls included central monitoring via CCTV and , enabling efficient oversight without onboard operators. Integration with Concourses A, B, C, and D provided seamless access across the airport's layout, supporting the shift to satellite terminal designs that centralized while distributing gates. Initial performance focused on handling baseline passenger volumes, with the system's fixed-guideway tunnels ensuring consistent service immune to surface traffic disruptions. Ridership grew steadily in the system's first decade, reaching 11 million passengers annually by 1980 amid rising demand at . To address this increase, three additional were added in the mid-1970s, boosting capacity on the loops without major changes. Further expansions in the included more to sustain operations, maintaining rates of 99.8% to 99.9% as reported from 1980 onward. These incremental upgrades ensured the SEA Underground met evolving needs while preserving its core role in intra-airport mobility.

Modernization and Rebranding

In the late 1990s and early 2000s, the conducted a comprehensive overhaul of the Satellite Transit System to address aging infrastructure and improve operational efficiency. The last major upgrade took place between 1999 and 2003, focusing on key subsystems such as . The modernization culminated in a full system replacement in 2004, introducing 21 vehicles—modified versions of the original CX-100 design—and the CITYFLO 650 (CBTC) system, enabling fully automated operations across the network's loops and shuttle. This upgrade enhanced system reliability by incorporating advanced moving-block signaling, allowing for smoother train movements and reduced headways. Service was restored progressively following the installation, supporting the growing passenger demands at Seattle-Tacoma International Airport. In 2022, the rebranded the system as SEA Underground, aligning it with the airport's broader "flySEA" identity launched in 2020 to streamline passenger navigation and boost marketing consistency. The new name, along with color-coded line designations (Green, Blue, and Yellow), was intended to increase familiarity for travelers amid ongoing airport enhancements. To further extend the system's lifespan, the Port initiated the SEA Underground Controls Replacement Project in 2023 as part of the comprehensive Upgrade SEA program. This initiative includes phased updates to train control hardware, software, and fiber optic infrastructure, aiming to optimize reliability and resource use without major service disruptions. As of 2025, Phase 1—involving fiber optic cable installation and tunnel lighting replacement—is ongoing, with full completion scheduled for 2027.

System Layout

Lines and Routes

The SEA Underground operates three distinct lines that interconnect the airport's Main Terminal, concourses, and facilities, enabling seamless passenger transfers entirely below ground level. These lines are color-coded for easy navigation and collectively form a compact network tailored to the airport's layout, with the and functioning as loops and the Line as a linear shuttle. The system totals approximately 1.7 miles in length, supporting high-frequency service to minimize walking distances. The Green Line, designated as the North Loop, forms a 4,100-foot circuit connecting the north end of the Main Terminal (adjacent to Concourse D), C, and the North . This route primarily serves passengers heading to or from northern , operating in both and counterclockwise directions to optimize flow within the northern airport sector. The Blue Line, referred to as the South Loop, traces a 3,700-foot path linking the south end of the Main Terminal (near Concourse A), B, and the South . It caters to traffic in the southern portion of the facility, similarly running bidirectional loops to facilitate efficient access to southern and reduce congestion at peak times. Complementing the loops, the Yellow Line provides a 1,000-foot direct shuttle exclusively between Concourses A and D, designed for rapid inter-concourse transfers without requiring a full loop traversal. This line operates back and forth along its short route, aiding passengers switching between southern and northern areas of the terminal.

Stations and Infrastructure

The SEA Underground system comprises six stations strategically located within Seattle-Tacoma International Airport to facilitate efficient passenger movement between concourses and terminals: Concourse A (near Gate A3), Concourse B (near Gates B20/21), South Satellite (below S Gates), Concourse D (near Gate D2), Concourse C (near Gate C11), and North Satellite (below N Concourse). These stations serve as key nodes in the airport's internal transit network, with each featuring a dedicated platform below the concourse level for quick boarding and alighting. The physical structures of the stations emphasize seamless integration with surrounding airport facilities, including direct connections to gates via escalators and walkways, as well as proximity to baggage claim areas in the main terminal stations. Platform designs incorporate wide, open layouts to accommodate high passenger volumes, with digital signage in multiple languages guiding users to their destinations. The total infrastructure supports six platforms across the network, enabling bidirectional travel on dedicated tracks that minimize transfer times. Supporting the stations is an extensive underground track system, consisting of 1.7 miles of tunnels housing two primary loops (north and ) and a connecting shuttle line. These tunnels were constructed during the airport's expansion. The design incorporates robust electrical power distribution throughout the tunnels to sustain automated operations, with recent upgrades including optic cabling for enhanced communication and control reliability. Construction of the tunnels and stations occurred as part of a comprehensive modernization program completed in 1973, which introduced the North and South Satellites alongside the transit system to link them to the main terminal. features, including accessible stairwells and ventilation systems tied to station platforms, ensure safe passenger egress during incidents, integrated directly with the airport's broader safety protocols. Overall, the stations and infrastructure prioritize durability and efficiency, with platforms elevated or at-grade within tunnels for level boarding and direct pedestrian links to and areas, enhancing the overall flow within complex.

Operation and Technology

Vehicles and Automation

The SEA Underground utilizes a fleet of 21 vehicles, developed by (now part of ), for its automated operations within . These rubber-tired trains are typically assembled in two-car configurations, offering a capacity of up to 80 passengers per train (40 per car) to efficiently handle peak airport traffic. The vehicles feature lightweight aluminum construction and advanced interior amenities, including seating for approximately 30 passengers per car and standing room for the remainder, ensuring quick boarding and alighting at the system's six stations. Automation is central to the system's efficiency, employing the Cityflo 650 (CBTC) system provided by Bombardier for fully driverless operation. This technology facilitates real-time train positioning, automatic collision avoidance through moving-block signaling, and precise stopping within inches of platform marks, optimizing dwell times to under 30 seconds. The maximum operating speed is 27 mph (43 km/h), balancing across the 1.7-mile network with safety in the underground environment. Automatic train supervision oversees dispatching, routing trains along the three lines (, , and ) with headways as low as 3 minutes during peak hours. The vehicles are electrically powered, drawing power from a rail-based electrical system for , supplemented by onboard for smooth and deceleration. This setup supports the rubber-tired design's low and vibration levels, critical for the subterranean tunnels connecting the main terminal to the North and South satellite concourses. The current rolling stock evolved from the original Westinghouse C-100 vehicles introduced when the system opened in 1973, with a comprehensive completed in 2003 that replaced the aging fleet with the modernized models and integrated the Cityflo 650 controls. This renewal extended the system's service life and improved reliability, handling approximately 21 million passengers annually as of 2019. As of 2025, the is undertaking the SEA Underground Controls Replacement project to update the automatic train control systems, enhancing capacity and recovery times.

Maintenance and Reliability

The SEA Underground, operated by the , undergoes routine maintenance managed by an in-house team of technicians, electricians, and engineers to ensure continuous service reliability. Daily monitoring occurs 24/7 through on-site duty representatives and support staff, including electronics technicians and automated control center operators, who address minor issues and perform immediate repairs. Monthly maintenance overhauls take place over weekends from Friday at 10:00 p.m. to Sunday at 4:00 a.m., allowing comprehensive inspections of the system's 21 cars without interrupting passenger service. During these periods, cars are uncoupled and transferred via a specialized table for detailed checks on air, electrical, braking, and other critical systems, lasting 30 minutes to one hour per car. Additionally, every 15,000 miles, vehicles receive enhanced preventative maintenance to preempt potential failures. The 's reliability is evidenced by a 99.95% uptime, supporting its 24/7/365 operational schedule that aligns with demands. protocols include rigorous system inspections, with any car failing standards immediately removed from service until repairs are completed, minimizing risks during high-volume periods. (CBTC) contributes to this reliability by enabling precise automation and fault detection, complementing the manual oversight provided by maintenance crews.

Features

Public Art

The public art integrated into SEA Underground stations forms part of the Port of Seattle's longstanding 1% for Art initiative, which allocates one percent of capital improvement project budgets to acquire and install contemporary artworks, a policy formalized in and rooted in the port's public art program dating back to the late . These installations, added during the airport's early modernization efforts, aim to enrich the passenger experience in the underground transit system connecting the main terminal to concourses and satellites. A prominent feature in the South Station is Nancy Blum's Flower Wall (2004), consisting of over 65 sculptures cast in aluminum and resin that depict vibrant floral motifs inspired by flora and international influences, spanning a 77-foot wall above the Loop entrance. The colorful, blooming forms blend natural abundance with urban transit aesthetics, drawing from both Western and Eastern botanical themes to evoke growth and environmental harmony. In the , Karen Ganz's Travelor (2002) comprises nine oil-on-canvas paintings arranged across a 110-foot undulating wall, portraying diverse travelers in contemplative scenes that capture the anticipation and transience of journeys. The hinged panels create a dynamic, choreographed flow, highlighting multicultural figures and everyday travel moments to foster a sense of connection among passengers. At the C Gates station, Cable Griffith's Cascadia (2019) is an immersive glass installation of 22 hand-painted panels using airbrushed enamel on glass, depicting landscapes, waterways, and mountains wrapping around the to the train platform. These artworks are intentionally designed to mitigate the monotony of waiting for s, thereby reducing perceived wait times through visual engagement, while infusing cultural depth and regional identity into the functional underground environment.

Accessibility and Capacity

The SEA Underground system incorporates comprehensive features to accommodate passengers with disabilities, in compliance with the Americans with Disabilities Act (ADA). All stations feature elevators and ramps for step-free access to platforms, ensuring seamless navigation for wheelchair users and those with mobility challenges. is installed along key pathways at stations to guide visually impaired travelers, while signage provides additional navigational support. The automated vehicles are fully accessible, with level boarding that eliminates the need for lifts or ramps on the trains themselves, allowing space for mobility aids, strollers, and other assistive devices. Audio announcements in stations and on trains provide arrival and departure information, enhancing usability for hearing-impaired passengers through visual displays as well. throughout the system is available in multiple languages, integrating with the broader to assist non-English speakers and diverse travelers. In terms of capacity, the SEA Underground handles approximately 16 million riders annually, based on 2015 figures when served over 42 million passengers overall. The system's supports peak throughput with operating in loops to manage high volumes, and vehicles briefly referenced here accommodate up to several dozen passengers each, including those with aids. Post-2003 upgrades, including a full replacement of the train cars and control systems by Bombardier, improved crowd flow during peak hours by enhancing reliability and reducing dwell times at stations.

Impact and Future

Ridership and Usage

The SEA Underground system accommodates approximately 40% of all passengers transiting (), serving as a vital link for transfers between the main terminal and satellite concourses. Historical ridership in 2015 reflected the system's integral role amid the airport's then-record total of 42.3 million passengers. Recent trends indicate robust post-COVID recovery, with estimated ridership surpassing 21 million in 2024—derived from the airport's all-time high of 52.6 million total passengers—demonstrating sustained demand for efficient intra-terminal movement. Usage patterns feature pronounced peaks aligned with major flight banks, when connecting passengers surge between concourses, and the system cuts walking times by up to 20 minutes for long-distance routes like those from Concourse A to the South Satellite. By facilitating seamless connectivity since its 1973 debut alongside the terminals, the SEA Underground has underpinned the 's expansion from roughly 5.2 million annual passengers at that time to today's record volumes, enhancing overall operational efficiency and economic contributions to the region.

Upgrades and Expansions

The SEA Underground Controls Replacement Project, initiated in 2023, aims to modernize the automated system's signaling and communication infrastructure while extending its operational lifespan. Phase 1, which began in the second quarter of 2023 and completed in the third quarter of , involved installing over two miles of and replacing tunnel lighting to enhance reliability and efficiency. Phase 2, starting in the third quarter of and projected for full completion by the third quarter of 2027, will upgrade the automatic train control systems, including computers, software, and video surveillance, at a total cost of $79 million funded by airport revenues. These upgrades align with the broader Upgrade SEA program, a $4.6 billion initiative by the to enhance airport facilities through 2027, including concourse expansions and improved passenger flow. The project supports the C Concourse Expansion, which adds four floors for additional amenities and is scheduled for substantial completion in mid-2026, ensuring seamless integration of the train system with expanded terminal spaces. Furthermore, the Controls Replacement ties into the Sustainable Airport Master Plan (SAMP), with environmental assessments approved by the in October 2025, which includes a new 19-gate terminal and envisions potential expansions of the automated to connect the main terminal, new terminal, and rental car facility. Future enhancements to SEA Underground focus on increasing capacity to handle projected passenger growth exceeding 60 million annually by 2030, driven by regional population increases and rising demand. Plans under propose possible line extensions via a new segment with three stations, improving connectivity and reducing ground transport needs through 2032. These developments prioritize , incorporating energy-efficient technologies such as LED lighting and fiber optic systems in line with the Port of Seattle's goals to cut by 50% by 2030, including integration and conservation measures across airport infrastructure.

References

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  25. https://www.portseattle.org/page/cascadia
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