Turning also doesn't involve slippage in trains. Many locomotives already have independently turning wheels, and the solid axles of train carts can also turn just fine (as explained in https://www.youtube.com/watch?v=f_-UJNETSlg for example)
The solid axle turning without slippage from that video works for regular inter-city rail. I think it doesn't work for rails with the tighter turning radii often required of inner city trams and similar rail transport.
