SEA Underground

SEA Underground

Company type	Automated people mover system
Industry	Transportation
Founded	1973
Headquarters	SeaTac, Washington , United States
Area served	Seattle–Tacoma International Airport
Owner	Port of Seattle

The SEA Underground, previously known as the Satellite Transit System (STS), is an automated people mover operating at Seattle–Tacoma International Airport in SeaTac, Washington, United States. Since its inauguration in 1973, it ranks among the earliest airport people mover networks worldwide, facilitating swift passenger transit between the Main Terminal and the North and South Satellite concourses.

History

Authorized in 1969 as the inaugural inter-terminal rail system in the United States, construction concluded in 1972 with an expenditure of $5 million, and public service commenced in July 1973. The project experienced setbacks caused by cost disagreements with Westinghouse Electric Corporation, the manufacturer. The overall initial expenditure totaled $14 million.

At launch, the system utilized nine Westinghouse-manufactured C-100 vehicles, each accommodating up to 102 passengers; an additional three vehicles were incorporated during the mid-1970s. Designed to operate at a peak speed of , it could transport up to 14,400 passengers per hour. Typical journey durations were approximately 3.3 minutes for loop routes and 1.8 minutes for shuttle segments.

In 1999, the Port of Seattle authorized a $142 million modernization program that encompassed upgrades to trains, control systems, power supply, stations, ventilation, and maintenance facilities. Completed by 2003, this refurbishment introduced advanced Innovia APM 100 vehicles alongside the Cityflo 650 CBTC automated control technology.

In March 2022, the Port rebranded the transit system as SEA Underground and implemented color-coded routes—Green, Blue, and Yellow—improving navigational clarity with matching station signage.

Layout and operation

SEA Underground functions exclusively within secure airport zones, serving six stations distributed across four gate concourses (A through D) as well as the North and South Satellite terminals. Each station is equipped with platform edge doors. The network consists of two loops servicing the satellite terminals and an interconnecting line running through the main terminal.

• The Green Line (North Train Loop) extends with stops at the north Main Terminal near Concourse D, Concourse C, and the North Satellite (N gates).
• The Blue Line (South Train Loop) covers with stations located at the south Main Terminal near Concourse A, Concourse B, and the South Satellite (S gates).
• The Yellow Line (Shuttle Train) operates over , connecting the northern end (Concourse D) and southern end (Concourse A) of the Main Terminal to link both loops.

The fleet comprises 21 APM vehicles, with about 13 typically active: two 3-car trains running on both Green and Blue Lines and a single car assigned to the Yellow Line. Nearly 40% of passengers passing through Sea-Tac utilize SEA Underground. Maintenance activities primarily occur during weekends; vehicles are transferred laterally via a specialized table to two dedicated servicing facilities.

The system boasts a reliability rate of 99.9%. Track upkeep on both Green and Blue Lines is generally scheduled overnight while a single train provides shuttle service. In case of power interruptions, onboard energy storage allows vehicles to proceed safely to the next station. Unlike many APM systems operated by their manufacturers, SEA Underground’s operation and maintenance are managed directly by the Port of Seattle, resulting in estimated cost reductions of approximately 40%.

Public art

During the 2003 refurbishment, several public art pieces were introduced. The south station within the main terminal showcases more than 56 sculptures composed of aluminum and aluminum/resin, created by Nancy Blum. Meanwhile, the north station features nine paintings centered around travel themes, crafted by Karen Ganz.

See also

• List of airport people mover systems