YOU CAN'T. There are no released Fritzbot waypoints for Railgun.
Unless Mal completes the train support he had to drop to work on ET:QW at Id then we waypointers could only make the bots act as support players in any attempt at waypointing Railgun. The bots would not understand the match objective is to move the train!
For the present if you want to play Railgun with bots use Bobots 4.XX.