On Tuesday, the San Francisco County Transit Authority [SFCTA]—a body composed of members of the Board of Supervisors and separate from SFMTA—decided on a $6.1 billion plan to connect Caltrain to the new Transbay Transit Terminal via a tunnel underneath Pennsylvania Avenue.

The unanimous SFCTA vote capped off months of planning and speculation on a proposal that stretches back more than a decade.

The city had considered three potential paths for the rail extension, but Tuesday’s Planning Department presentation to SFCTA favored the Pennsylvania option.

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“For most of its lengths the Pennsylvania alignment can be bored, just like the Central Subway was,” Planning Director John Rahaim said Tuesday, emphasizing that this would result in minimal street-level disruption throughout construction.

Among the proposed Pennsylvania plan benefits highlighted in accompanying documentation:

[It] uites Mission Bay with the city, removes barriers, maintains access and mobility for critical life - saving services, and avoids a long, deep trenching of streets to maintain east/west connections. Provides for potential increased operational capacity via underground expansion of the 4th/Townsend station to allow for additional storage or staging opportunities for Caltrain. Could be built an estimated four years sooner and at a significantly lower cost than the Third Street alignment.

According to an SFMTA assessment from June, putting the route beneath Third Street would have cost $9.3 billion and not finished until 2037.

A competing third plan would have cost just $5.1 billion and been finished by 2026, a year earlier than the Pennsylvania Avenue route. But it would also have meant letting trains run at surface level through busy traffic.

Rahaim called the vote for the Pennsylvania alignment “a 100-year decision.”

At the hearing, Supervisor Malia Cohen, whose district most of the route runs through, said that community feedback thus far was “mostly negative.”

But she compared the extension to BART and the Golden Gate Bridge—two other huge transit plans that were contentious at the time but in hindsight no-brainers—and predicted that the route would garner popular support in the long run.

Supervisor Sandra Lee Fewer chastised SF Planning for not doing more community outreach in neighborhoods where the trains won’t run, noting that the plan “effects people in SF in all districts” and that future commuters “have not been given an opportunity to even weigh in.”

Despite these minor reservations, the plan carried unanimously. Note that approval from many other agencies is needed for the full proposal to move forward.