That flexibility is also a reason for building bus routes: they can be re-routed as needed due to construction or changing rider patterns, buses can be replaced

That flexibility is also a reason for building bus routes: they can be re-routed as needed due to construction or changing rider patterns, buses can be replaced (improved) without having to retrofit the entire system, new ones can be added where needed, old ones can be removed where not, etc. Central planners might like the idea that they can choose which neighborhoods will succeed, but count me among those who think they can F themselves. Buses work. They're cheaper than every kind of rail. They're more flexible. But planners and meddlers want control.