What's the incompatibility? It's 4' 8.5" standard gauge with 25KV AC catenary. (BART is 5' 6" with 1KV DC third rail, both of which are unique to BART.)
The construction process does seem to be insufficiently standardized. There's all that falsework and ground paving. Compare the way it's done in China, with standardized bridge sections placed on pylons by a very specialized machine.[1]
They're insisting on a 50" above rail platform height for starters. Higher than anywhere else in the world and incompatible with all other rail in the US let alone CA. There is no comprehensive planning between the different agencies running commuter and regional rail and CAHSR to work towards common inter-operable standards. This should be state wide, if not across the whole west coast. (We really need state wide rail planning.)
50" is about normal. US northeast corridor is 48 inches; China is 49.2". Japan's Shinkansen is 49.2" EU high speed rail is much lower, requiring bogies between cars.
Eurostar does, and some TGV trainsets do. But some other high speed rail trainsets in Europe don't.
If the floor is below the top of the wheels, there has to be a narrow section at the car ends. The wheels have to go someplace. The lost space can be minimized by sharing a bogie between two cars.
(If the floor is below the axle line, as with ultra low floor trams, it's really complicated.)
Eurostar is the exception to every train. It's a monster of complexity because it has to work on three systems. It also does not have 550mm platform height. I think it has 750?
> some TGV trainsets
But not because of the platform height but because of stability in case of a crash. The TGVs with bogies between the cars has the same general height setup as the traditional ones.
> (If the floor is below the axle line, as with ultra low floor trams, it's really complicated.)
Sure, but that's hardly relevant here. Trams are generally set up completely different. Vienna for instance has the ULF tram and it's being phased out because such a low floor setup is just generally a bad idea for many different reasons. The new rolling stock will be Bombardier Flexity.
Eurostar is 760mm. The EU TSI requires 550mm or 760mm; the UK rail network has a blanket exemption excluding it from that (platforms are more-or-less universally 915mm in the UK).
The construction process does seem to be insufficiently standardized. There's all that falsework and ground paving. Compare the way it's done in China, with standardized bridge sections placed on pylons by a very specialized machine.[1]
[1] https://www.youtube.com/watch?v=PbaD2-2Ktwc