Above a certain speed wheels present huge issues, which means you need to change your drive mechanism and suspension mechanism. Also, drag is a huge killer of

Above a certain speed wheels present huge issues, which means you need to change your drive mechanism and suspension mechanism. Also, drag is a huge killer of efficiency and hence overall economics. Even at 500kph the drag on a train at sea level is enormous. At 1000kph it's 4 times bigger again, and you've got the sound barrier to worry about. Trying to do the same with multiple small capacity trains is astronomically worse. So, better to have reduced drag... and by now you've basically got the hyperloop.