I'm surprised the article didn't mention that BART is the only train in North America on the uncommon 5'6" gauge. This is another factor making parts and equipment more difficult to source.
If SF used standard gauge rail, it's not like they could just buy some surplus rail cars, put some seats in them, and call it a day. Subways are generally lower to the ground because it improves stability and reduces roll. They corner better. I'm not sure that any subway system uses regular old heavy rail train cars.
And besides, if you're ordering thousands of cars for replacement from bombardier (or whomever), you can order spare parts while you're at it.
I don't think most parts and equipment are swappable anyhow. Maybe tracks
themselves. But as far as the trains are concerned, you're pretty much
buying all of the trains bespoke from the manufacturer.
Muni runs all sorts of heritage equipment from around the world on the F, but the PCCs which make up the majority of the fleet were designed as a standard car.
If SF used standard gauge rail, it's not like they could just buy some surplus
rail cars, put some seats in them, and call it a day. Subways are generally lower
to the ground because it improves stability and reduces roll. They corner better.
I'm not sure that any subway system uses regular old heavy rail train cars.
Muni's current trams are based on an off-the-shelf Siemens design. The problem with BART isn't just the axles, it's the entire propulsion system, it's the wacky loading gauge, etc. BART can't even get away with a modified off-the-shelf system.
And besides, if you're ordering thousands of cars for replacement from bombardier
(or whomever), you can order spare parts while you're at it.
No-one will have a 5'6" track replacement train for hire.
As far as the spares are concerned... Yeah, I'm not really sure what to tell you. It's not a problem limited to BART. Most subway cars in the world are produced by Kawasaki, Bombardier, etc. Even still, I wonder how much other cities with standard rolling stock get to reuse parts in maintaining subways? Perhaps you know.
I don't doubt there are other random parts difficult to find or source. Especially from a system that was originally built before electric train standards fully formed. The cost of innovation is maintenance.
The cost of innovation is maintenance.
Muni (with a lot of effort) can run a fleet of trams that are 70–100 years old. They've modernized the propulsion where necessary and made whatever other changes were needed for compatibility. Part of the reason that's possible is that Muni runs a 600 volt system which is common worldwide (although largely supplanted by 750 volt setups). These antiques run in revenue and charter service all over the city and yet BART can't find windows for its cars that were built to its specifications.
BART ordered brand new bespoke trains from Bombardier and still uses a bespoke traction system (1000 V DC). Factor in BART's one-off driverless cable car and their standard gauge diesel line. In 15 years when stuff starts wearing out, which agency do you think is going to have a more difficult time with procurement and maintenance?
Caltrain is "heavy" rail, sure. Muni isn't. When they were new the PCCs were used globally (including in San Francisco) and serve as the basis for a number of modern designs. BART set out to be the sexy anti-rail and that's the problem. Fifty years on and they've doubled down on bespoke gadgetry.
High winds, lighter cars.
Even standard gauge trams have no difficulties navigating curves that are much tighter (25 m, down to 10-ish m on some legacy systems) than anything you'd seriously consider when building on a new rapid transit type rail system (normally at the very least a three-digit metre figure, ideally something more like 200 or 300 m).
The only thing that directly scales with the track gauge is the length difference between the outer and inner rail in a curve and the minimum radius down to which the self-steering effect of conical wheels can still compensate this length difference. This means that with a larger track gauge you might get a little more wear and tear for the same curve radius, but a moderate widening of track gauge (e.g. BART is approx. 16 % wider than standard gauge) will equally not cause a dramatic effect here.
In the best case, with standard gauge self-steering might work down to 150 m, which in the case of BART's track gauge would turn into 175 m, which is still rather on the tight side for a rapid transit-style line (and as originally built, BART didn't use conical wheels anyway, so they didn't benefit from self steering in curves regardless of radius used).
The bigger factor in terms of curve radii is the length of your vehicles, how much vehicle throw you're prepared to accept in tight curves, and possibly whether you want to have trains with open gangways, so passengers are able to move between individual vehicles.
https://en.wikipedia.org/wiki/Standard-gauge_railway
I doubt that BART has super-special stability issues necessitating 5'6".
Unless they are a train geek they wouldn't say that, and probably wouldn't even know it. To people into trains it is known as Indian broad gauge, but most people won't know about such details, nor do they have reason to care. Though I would hope the operators know this - it makes it easier to order trains when you can use the industry terms.