Yes
>Is track maintenance a fixed cost (this has to be done more often than the trains themselves)
Yes
>Equally, are signalling upgrades (for example) fixed costs?
Yes
Fixed costs are the costs that don't go away when the passengers go away. You still have to amortize the trains that don't go on the tracks. You still have to do track maintenance even when you're not running trains (though, yes, at the very margin there could be some small rate adjustments). When you want to upgrade the signals, it's basically the definition of a fixed cost, because you do it once and it's done.
>And National Rail isn't replaceable at all with bus/cars/planes. You really underestimate the number of people which commute >1hr into London (100km+). There is just no way to do that journey by car or bus. It would turn a ~1hr commute into a 3hr _each way_ and that's not even considering the complete lack of parking OR the fact suddenly the roads would be at (even more) gridlock with many multiples of commuters.
I don't want to speak to National Rail or British Rail that preceded it. I want to stick to the transit system that I know well.
My point here isn't that money shouldn't be spent on "getting things back in shape" here is where I waffle on the "pay for fixed capital costs and mostly have the marginal variable costs covered by the marginal rider." If a system needs the occasional cash infusion, I'm fine with that, as long as it comes with new leadership.
My concern here is that, in the Bay Area, many, many people are eager to pay $25 for a Waymo to pick them up (they are NOT cheap) while Muni costs $3 (a near 10x increase in cost). When folks are willing to pay that much of a premium, then something is very wrong with the transit system. Muni has had zero enforcement of their code of conduct for decades. When you have a system that are large section of the populous actively avoid when it's perfectly convenient, then something is very wrong with the system.
When I see BART stations that look like abandoned parking lots surrounded by single family home sprawl, then it doesn't surprise me that the system is not sustainable. The stations that may get removed are all in areas that require people to drive, to then take the train, instead of the cities zoning density and retail around the train stations. When I yell at the occasional people smoking in BART stations and I go to tell the station attendant and get a shrug back -- even when we are paying for them to have their own police force -- that's why they are failing. These are political choices that BART has made in how they operate their service
These systems aren't even doing the bare minimum in providing a reliable pleasant service, so people stop using them, and that makes sense. The entire point is that these services should be relatively inexpensive to operate because of economies of scale, but when you don't actually make people pay, when you don't actually ask people to behave like responsible adults, when your running the service like a failing business then we should expect the service to fail, and when it does, when bailouts are needed, they should (and often do) come with strings attached. BART now has gates that stop most turnstile jumping... and they were forced to be installed by the state of California as part of their second bailout. The reason I'm harping on having variable costs attached to ridership is exactly because the systems needs to be forced to respond when a sizable amount of people no longer find the service valuable.
This is about sustainability, because the marginal tax dollar is better spend on something like providing people with the healthcare they need than it is providing people a bus service they're not even willing to actually use.