Link21, the nascent megaproject to beef up the Bay Area's passenger rail network, features at its core a new underground transbay passenger rail crossing between Oakland and San Francisco. One of the major dilemmas facing this program is what kind of rail service to put in that new tunnel. The choice is posed between BART (understood as wide-gauge single-level rolling stock) and regional rail (understood as standard gauge FRA-compatible rolling stock). One or the other, but not both.
This is a false choice!
Where would this new standard gauge BART line go? From Oakland, it would continue south via existing and under-utilized standard gauge rail corridors to serve the (re-gauged) Dublin / Pleasanton branch, which could be further extended to serve the Altamont Pass (hello, Valley Link) and points beyond. Long-distance trains, provided they have a pantograph, could run through from the Central Valley straight into downtown San Francisco.
Link21 planners should stop promoting this specious choice between BART and regional rail. They don't need separate tunnels, and the dogmatic belief that they do will inevitably result in terrible planning decisions.
Small print regarding the graphic: the single tunnel bore (of two) is drawn rigorously to scale. The BART broad gauge version is 250 inches (almost 21 feet) in external diameter, taken from a VTA drawing. The regional rail version is 8 meters (a bit over 26 feet) to accommodate taller bi-level trains. The overhead conductor rail, of the same design used by Caltrain in its San Francisco tunnels, is 18 feet above the rail. The BART standard gauge train is a Stadler KISS with the same dimensions as Caltrain's, but a different color of paint as previously seen in this post.
Note an alternate configuration is also possible with a 40-foot external diameter single bore tunnel.