Make the first one faster and cheaper alternative to a congested route and I don’t see how it fails. I think the perceived failures are when they measure “success” by fare revenue or something stupid like that. benefits are way more diffuse.

But if the existing routes are not congested, then it’s going to be hard to offer massive time savings.

Even in the early (private, not really ‘networked’) days of london commuter rtooailways , the ones that ‘failed’ were mostly after trams and (motorised) buses came along and undercut them and offered a cheaper more convenient service. But generally speaking most of them , including the first ones (metro, district, casl etc) are more or less still operating today in one form or another. Of course the trams and buses got fucked by cars in the 60s, so that brought the trains back into it. And the trains and trams and autobusses also drove out the horse drawn buses before that…

I guess everything fails when the most efficient technology moves on. I’m sure the hyperloop will be killing off underground electric railways any time now.

I think anywhere you get enough density in people and activity then its hard to fail at public transport, apart from just not trying or not trying hard enough; political failures not technological ones. Low density housing and long commutes though, that’s a harder problem though.