Menu Home

SEA Underground Controls Replacement

Status: In Progress

The SEA Underground Controls Replacement Project updates and extends the life of the airport’s train system to keep people moving for years to come.

Project Overview

The 1970s was a decade of expansion and redevelopment at SEA Airport. A new central terminal structure encased the original 1949 building. New skybridges connected the terminal to a new multi-level parking garage. Also new, and the first of its kind in the nation, were a pair of subway loops and custom trains shuttling travelers to the S and N Concourses (formerly North and South Satellites).

Since opening in 1973, millions of passengers a year have relied on the SEA Underground to reach their gate. The last major upgrade was in the early 2000s, and the system is due for a refresh. This Upgrade SEA project will update important train components and software over the course of two phases, improving the customer experience and optimizing airport resources. The project is scheduled to complete in 2027.

Phase 1: Install fiber optic cable and replace lighting in the tunnels
Status: Ongoing

  • Crews are installing more than two miles of fiber cable throughout the underground tunnel system and airport. The cable will support new communication systems being upgraded in later project phases. 

Phase 2: Upgrade automatic train control systems like computer servers and software, and video 
Status: In Design and Factory Testing

  • Phase two of the project will upgrade the controls systems both in the office and on the trains to help prolong the life of the train system and maintain trip reliability.

Phase two of the project will take place over multiple years as systems are updated and tested for safety. During that time, there will be occasional and temporary overnight closures while work is completed. Passengers will be directed to use shuttle buses to reach their gates during these closures. Check here for the latest construction impacts.


Phase 1 Construction began in Q2 2023 Estimated completion Q3 2024
Phase 2 Installation estimated to begin Q3 2024 Estimated completion Q3 2027

Estimated Project Costs

Total: $79 million

Funding for the project will come from a combination of Airport Development Fund and future revenue bonds. As with virtually all airport projects, no tax-payer dollars will be used to fund the project.

Related to SEA Underground Controls Replacement

Back to Top