I mean, I guess I'm mostly just thinking about high density places. I don't think you need light rail in less dense places.
I don't know about the preciousness about buses. Short distance buses make lots of sense. Long distance buses make medium sense. Medium distance buses in hyper dense places as commuter options are kinda a mess. And you're trying to solve medium-distance needs for the most part. (I get that it's going to be a consistent cost, but that kinda part of public goods and such. Some transit infrastructure doesn't necessarily directly pay for itself, but the indirect benefits carry value)
What are the places it's pushed where it won't work?
Phoenix is a good example of where it won't work... But it doesn't stop them from trying.
Full disclosure: the author of the blog I'm about to link is pretty strongly libertarian, so obviously he comes at this from a certain point of bias. However I strongly recommend reading some of his posts with the Phoenix Light Rail tag. It's some really good insight.
https://coyoteblog.com/coyote_blog/tag/phoenix-light-rail Specifically, though, this is a really good one: https://coyoteblog.com/coyote_blog/2014/10/phoenix-light-rail-update-we-spent-1-4billion-to-reduce-transit-ridership.html
If you look at Phoenix, they were showing consistent increases in transit ridership for a decade. As soon as they built light rail, transit ridership--in a city that's been growing like gangbusters--immediately stagnated. Bus ridership went down and light rail went up, at almost a 1:1 number. Was that worth it?
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The issue is a lot of places like to think they're high density enough for light rail, but... They're not. Every medium-sized city has people who fetishize light rail. But rarely does it actually work out.
You get these enlightened city planners who think they can "revitalize downtown" by connecting the suburbs to the city center via light rail. Of course, in a lot of cases those suburban workers work in... The suburbs. Light rail, rather than being a real commuting option, tends to be what drunks use to get downtown to party so they don't have to drive. That's not a bad thing (I've done it), but hardly worth the billions that cities throw at it.
The poor largely don't need medium-distance travel. Usually a big part of being poor is working relatively low-skill jobs, which doesn't require commuting "downtown" every day.
Light rail is a great thing for middle-class folks who have nice houses and shiny vehicles they leave at their park & ride to feel good about themselves for taking "transit" to work and being green. But it's a really expensive solution for that "problem", and the cost of that solution then tends to crowd out meaningful transit options for those who need it most.