Technology is not important. The reasons to prefer trains and subways are not the technology so much as most buses are running in the same lane as cars and so they have the same limitations. A bus in a truly dedicated lane would work just as well for most purposes, if only we cared about them.

There are a couple advantages to trains/subways over buses, but for the most part they don't apply.

They can be longer: have more riders, which is only important when they come every 5 minutes. When the bus isn't coming every 5 minutes you should address overcrowding by adding more buses which makes the whole system more convenient for the riders.

They run steel on steel which is more energy efficient. While this is a factor, a bus is so much more better than cars we shouldn't count it.