Bart picked India broad gauge. While it isn't common in the US, it is common in the world (mostly India as you can tell by the name) and so there are plenty of off the shelf train components available in the world market (though the US often has goofy Buy American rules that get in the way). Even at that the only difference is the axles which are easy for any manufacture to substitute on an otherwise off the shelf train.

Don't get me wrong, I think BART should have just gone with standard gauge. However the gauge isn't goofy and is not a source of big problems.