Even if you think what you would say is obvious, please add. This is genuinely something I think makes sense regarding local bus routes given the longevity of light rail and how infrequently routes change, but I also suffer from confirmation bias, so I’m hoping for reasons this would be a terrible idea but obviously would prefer reasons it would be an even more amazing idea than I thought.
Depends on the road layout; if it’s a long straight road then light railway makes sense. It’s less maintenance, easier to operate, can move unhindered because it doesn’t get stuck in traffic (edit: provided they don’t share the roads).
For spaghetti road layouts though, I don’t see the benefit, but I could be wrong since I’m no expert.
There is no reason rail has to be constrained to the road network. Eminent Domain is literally for things like this.