This is a major obstacle to physical merger of BART with other regional transit networks such as CalTrain and SF's and SJ's light rail.
What was the reason for this choice? Over the years, I've heard quite a few:
- A broader gauge allows higher speeds (but note--BART tops out at 80mph, while TGV's and Bullet trains go 3x as fast on standard gauge track).
- It makes room for larger electrical motors (but BART car trucks are build with frames inside of the wheels--outside framed, standard gauage trucks would probably leave just as much room).
- MetroRiderLA's Wad mentions a reason I hadn't heard before: a broad gauge would make trains more stable, and able to run across the Golden Gate Bridge. But he notes that Marin had opted out of BART long before the system was built, or even designed.
- Compatibility with Indian railways (just kidding, but it makes as much sense as anything else).
None of these really add up. My best guess for BART's eschewing of standard gauge is just:
- Not invented here.
You can find out all you want to know about railway gauges and more in this Wikipedia entry