The weird gauge is indeed weird, but is generally overstated as a problem for BART. There’s not much to be gained by changing it system-wide, and it would be at great cost.

It’s sometimes claimed to be a driver of costlier rolling-stock acquisition, but this is dubious. There are many track gauges around the world, and not a lot of evidence that nonstandard ones cost dramatically different amounts. And even if bespoke gauges did cost more for economies-of-scale reasons, BART’s gauge is hardly unique: it’s the same gauge used in India, and just a couple mm from the one used in Iberia and much of Latin America (in practice they are demonstrably interchangeable). So there is a huge amount of rolling stock being manufactured for this gauge. It doesn’t require reinventing the entire production process or something.

The other claimed benefit to standardizing BART’s gauge that I sometimes hear is interoperability with other systems, but this is very difficult for several larger reasons, and also not necessary or desirable. For one thing, BART also has a nonstandard wheel profile and corresponding rail profile, so you wouldn’t just need to move the rails closer together, you’d actually have to replace them all. Beyond that, mainline rail systems have tougher crash standards than fully-separated metro systems (even after recent reforms), so you couldn’t run BART trains onto mainline tracks unless you bought heavier (and more expensive) rolling stock than what BART and other metro systems normally buy. Another issue is that mainline rail systems use different signaling and electrification (if they even have electrification) systems, which would require either an incredibly expensive overhaul or the use of multi-current/multi-signaling trains, which again increases the cost. Also, mainline rail is generally not grade-separated like BART, which would need to be changed at great expense, or BART would have to accept reduced reliability due to grade-crossing conflicts (this matters less for infrequent regional rail, but is huge for BART which must timetable dozens of trains per peak hour through the Y-junction in Oakland). Stations would also need to be harmonized to the same platform height (BART’s is different from any other system in the area) and length (stations would need to match BART’s extremely long trains).

Interoperability also doesn’t offer much benefit. Usually I hear about this in the context of Caltrain: instead of having these fragmented systems, wouldn’t it be nice if BART served the peninsula? I agree! We can do this by merging the agencies and putting new logos on the trains, without worrying about replacing rails and redoing the electrification that Caltrain is just finishing up with. I’m only slightly oversimplifying. We should continue iteratively upgrading Caltrain into true rapid transit with things like BART-style level boarding, all-day high frequencies and regular intervals, and grade separation. And we should extend what is currently known as Caltrain downtown and then across to the East Bay[0], providing good transfer opportunities with current BART. If this is done well, it should all feel like one regional rapid-transit system to the rider[1], much like New York subway and Berlin U-bahn riders usually don't even realize that they’re actually using multiple incompatible systems. At the end of the day, good integration and high service quality are what matter to the rider, not the underlying technology.

I think eBART contributed a lot to the idea that BART’s gauge is a big problem, since this weird DMU thing was sold under the premise that “normal BART” was too expensive. Really the things that were (allegedly) too expensive for this low-ridership exurban tail were electrification and the huge stations necessitated by BART’s extremely long trains. Those factors would've still been there even if the rails matched up.

So was it pointless and dumb for BART to use a nonstandard track gauge? Yeah. It was part of the marketing for a “space age” system that wanted to be seen as different from ye olde train. But is it really a huge problem that’s worth correcting? Nah. It’s only worth changing part of it if you wanted to convert a segment of track from mainline to BART or vice versa—and the cost of the track stuff would pale in comparison to other factors (electrification, signaling, stations, grade separation).

[0] https://caltrain-hsr.blogspot.com/2021/09/down-tubes-with-dt...
[1] https://caltrain-hsr.blogspot.com/2023/03/the-false-choice-o...