The Ballard Link Extension (BLE) is one of the most significant upcoming transit tunnelling projects in North America. Planned as part of the Sound Transit 3 regional expansion programme, the scheme will extend Seattle’s Link light-rail network by approximately 7.7 miles (12.4 km) and includes the construction of a new underground transit tunnel beneath downtown Seattle.

While the project remains in the planning and environmental review phase, the scale of the proposed tunnelling works and the anticipated procurement timeline make it a key scheme to watch for the tunnelling industry.

Project Overview

The Ballard Link Extension will connect the neighbourhood of Ballard in northwest Seattle with downtown and the wider regional light-rail system. The extension is expected to serve several key districts including Interbay, Uptown, South Lake Union and the city centre.

The project is being delivered by Sound Transit (Central Puget Sound Regional Transit Authority) under the Sound Transit 3 (ST3) expansion programme.

Current programme estimates place the overall cost of the Ballard Link Extension in the range of US$20.1 billion to US$22.6 billion, reflecting the complexity of the downtown tunnelling works and multiple underground stations.

Once complete, the line will provide additional capacity to the regional transit system while relieving pressure on the existing Downtown Seattle Transit Tunnel.

The New Downtown Tunnel

A central component of the scheme is the construction of a new light-rail-only tunnel beneath downtown Seattle, designed to allow future service expansion and operational separation from the existing transit tunnel.

The proposed downtown alignment is approximately 3.3 miles (5.3 km) in length and will run beneath the city centre, linking the Chinatown–International District area with South Lake Union and Seattle Center.

Several new underground stations are planned along the route, including:

• Chinatown–International District
• Midtown
• Westlake (expanded interchange)
• Denny
• South Lake Union
• Seattle Center / Uptown

Due to the dense urban environment and existing infrastructure, some of the stations are expected to be constructed at depths exceeding 40 m (around 135–140 ft).

Engineering and Early Development

Early programme development and engineering work has already begun. Sound Transit awarded a US$285.9 million project development and engineering contract to HNTB, covering preliminary engineering and corridor studies.

Extensive geotechnical investigations have also been undertaken along the alignment, including numerous boreholes to better understand subsurface conditions in the downtown corridor.

These early investigations will inform the final tunnel alignment, station construction methods and procurement packaging.

Programme Timeline

The project remains in the environmental review and planning phase, with several key milestones ahead before construction procurement begins.

Current indicative timelines include:

• Draft Environmental Impact Statement (EIS): expected in 2026
• Final Environmental approvals and Record of Decision: later in 2026
• Final design phase: approximately 2026–2030
• Construction start: around 2028
• Planned opening: around 2039

The long programme reflects the complexity of building a major underground transit corridor through Seattle’s dense downtown core.

Likely Procurement Strategy

Although procurement details have not yet been finalised, Sound Transit has historically delivered large transit infrastructure schemes through major design–build contracts, and a similar approach is expected for the Ballard Link Extension.

The works are likely to be divided into several major contract packages, potentially including:

Downtown Tunnel Package

• TBM drives beneath downtown Seattle
• Construction of underground stations
• Cross passages and ventilation facilities
• Portal structures

North Seattle / Interbay Guideway

• Elevated structures and guideway
• Surface or elevated stations

Rail Systems

• Trackwork
• Power supply
• Signalling and communications

Individual design–build packages on comparable US transit tunnel projects have ranged from US$1 billion to US$4 billion.

Contractors to Watch

Although tendering has not yet begun, several international contractors with strong track records in US transit tunnelling are likely to monitor the opportunity closely. Companies frequently involved in similar projects include:

• Dragados USA
• Tutor Perini / Frontier-Kemper
• Kiewit
• Skanska
• Obayashi and Shimizu (often participating in joint ventures)

Major TBM supply contracts are also expected once tunnelling packages advance to procurement.

Challenges and Uncertainties

Like many large infrastructure programmes, the Ballard Link Extension faces several challenges that may influence procurement and delivery.

Cost escalation across large US transit projects has placed pressure on the programme budget, and discussions have taken place around potential design changes to manage costs. One topic under review has been whether the proposed second downtown tunnel should proceed in its current form.

In addition, the siting of stations in the Chinatown–International District area has generated significant public debate, which could influence alignment decisions.

These factors mean the final project configuration may continue to evolve during the environmental approval process.

A Major Future Tunnel Project

Despite these uncertainties, the Ballard Link Extension remains one of the largest planned transit tunnelling projects in the United States.

With environmental approvals expected in 2026 and procurement potentially beginning around 2027–2028, the scheme is likely to become a major opportunity for tunnelling contractors and suppliers in the coming decade.

If delivered as currently planned, the new downtown Seattle tunnel will play a central role in expanding the region’s rapid transit network while providing a significant addition to the pipeline of large-scale urban tunnelling projects in North America. Further information on the Ballard Link Extension project, including maps, project documents and timeline updates, can be found on the Sound Transit website: www.soundtransit.org/system-expansion/ballard-link-extension. 10/26.