"Elevated" "subway" systems are generally extremely expensive to build and operate, perhaps the most expensive approach, though it does look elegant in artists' drawings. It's not surprising that those who don't have to pay for it like such systems but that those who would foot the bill are less enthusiastic...

In many of the areas of the US the problem is low population density and lack of centralization - there isn't a any "downtown" to concentrate the workers. It's not that hard to move workers to and from dispersed suburbs to the big factory in town but in a city like Houston with no central work area and spread out over more than 3,000 square miles it's a lot harder to come up with a system useful to very many people.