Bus Rapid Transit is another option that could be amazing (while being much cheaper to implement), but it falls short for the same reason as trains: they require dedicated infrastructure that complicates driving, and complicating driving is political suicide.
One of the things I found when advocating for transit was that BRT cost savings in the US almost always come from reducing quality at stations, which loses public support faster than you save money. I found that voters are usually willing to spend far more on trains than on BRT, in excess of any savings.
Wow; that's surprising.
People vote with their gut. Their gut tells them that buses are terrible and trains are generally good. They're right.
BRT is mostly "you get what you pay for" - cheaper at a cost of lower capacity. Given relatively low density of US cities - that might be the right tool tho.