But why make it a traditional train? Just build a lightweight train if it gets you all of the hyperloop's cost advantages without the additional costs and complexity of a tube?
The reason we have to run long, heavy trains is it's unsafe to run two passenger capsules at 300kph a minute apart. I find it hard to believe regulations will permit that even when they're in a tube.
Traditional trains have wheels and engines, which are quite heavy. And with a traditional train you're trying to minimize losses due to air resistance, which is not an issue in a low pressure tunnel.
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.
