Ok, but if you take a bus and do all of the things I mentioned to it, you end up with a "rubber tire metro", which does exist, and is used in Paris and Mexico c

Ok, but if you take a bus and do all of the things I mentioned to it, you end up with a "rubber tire metro", which does exist, and is used in Paris and Mexico city, at least. These are basically indistinguishable from a passenger perspective to a train. The smoothness of trains is really out of necessity. If the tracks weren't smooth, the train would derail.