I've always wondered why they've been so generally unsuccessful; conventional elevated rail works fairly well, and at least in theory they should be much cheaper and less obtrusive.
Sure, but there's no reason that _has_ to be the case. Like, there've been attempts at standards; the volume is just never really there to make any of them _the_ standard.
Underground metro systems mostly started off as deeply weird and proprietary, as well (look at this one, weird gauge, tiny little trains, originally _cable_ operated: https://en.wikipedia.org/wiki/Glasgow_Subway), but nowadays they're _fairly_ standard, say.
Yes they work but not better then normal trains and worse in some ways. Qnd in terms of sharing infrastructure maintaince, signaling and so on its worse.
I've always wondered why they've been so generally unsuccessful; conventional elevated rail works fairly well, and at least in theory they should be much cheaper and less obtrusive.