BRT is a very good use case for this, where if rail is too expensive per mile to maintain from ridership you can still have a fast connection with 15 minute headways or better.

It’s popular because it’s cheap and fast to implement, so much so that latin american cities all over have been having massive success with them despite a limited budget. Unfortunately where I’m at the political fight has gotten so bad that they are cancelling bus lanes with lame duck reasoning such as “the paint is too expensive to maintain” and “we need more lanes for general traffic to sit in traffic in”.