Because a train has 2 fronts. If it has a front and a rear, it would take up a lot of track and space for the train to turn around. By having 2 fronts, when the train gets to its destination, the conductor just walks to the other end and he’s ready to go.
If you played the Simpsons tapped out, you can build a monorail, which is a train that’s been elevated. Anyway, if you build a track that goes from one end to the other, the train will go to the end then reverse itself but the train has 2 fronts.