The effort to engineer new train tunnels across the San Francisco Bay is gaining traction after the planning body responsible for overseeing the massive proposed Link21 infrastructure project unveiled conceptual maps detailing a key segment of the rail network on September 21.

The maps offer a look at two separate plans that would either divide the tunnel along regional/local service lines or combine them both into one corridor that connects Oakland and Alameda County to the Salesforce Transit Center and Mission Bay. They also included references to a "possible extension" of the BART system leading away from downtown to the west.

The details released last week hint at a number of key future projects that have made headlines in the region for a number of years. Other than enhancing links to the larger regional Caltrans system, plans show an addition of new BART service routes along the contested I-980 corridor, along with four added stations in Alameda, East Oakland, the city’s downtown, and Jack London Square, which had been previously floated as the site of the Oakland A’s new baseball stadium.

The Link21 name refers to the scope of the 21 counties "megaregion" its combined rail network will enable. Its final vision would connect the cities of Santa Cruz, Salinas and Monterey to the south with the Sacramento region and communities in the San Joaquin Valley for the first time. 

A report from The San Francisco Examiner indicated the project's estimated initial cost to be $29 billion. Link21 says this total will likely be supplemented by "various value capture" strategies. Once final approval is granted, construction could last around a decade before the project receives its finishing touches in the year 2039.