There are/should be quite a few benefits to having a rail system in the game.
1. Trains obviously
2. Transporting large weapons such as artillery, and vehicles too. Would make for amazing missions to protect the train etc.
3. Invisible rails on the roads available in the editor so that players can force the AI in cars to travel on the rails in the direction they need them to travel in. Wont work if it's a human player driving the vehicle.
May help without having the need to use unitCapture command etc.
Would have to be used with a command of course.
There are probably more benefits to rails for cutscenes and of course gameplay as well.