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.