This is an easy problem to solve. You just have the paint mechanism at the back of the vehicle so it paints after the wheels have moved over the track. Kind of

This is an easy problem to solve. You just have the paint mechanism at the back of the vehicle so it paints after the wheels have moved over the track. Kind of like the reverse of the plough at the front of the train that some networks (not U.K.) have to clear the track before the wheels pass over it.