To try and answer your question:- Do you think one horse could pull along your train? I reckon so. So a one horsepower motor will pull it along at 2 mph. Your problem is not in the motor, but in the gearbox. If you gan only get a 5 hp motor, juggle the gear assembly to get the right balance between speed and power. How much reserve power do you want to keep in hand?
Then there are different types of motors. Some are good at starting off under fine control at low speeds, others are only desiged to run at high speed through a gearbox. Some will run continuosly without overheating, others wont.
You have really have a Junkyard Challenge to see what motors you can get hold of from wheelchairs, or washing machines, or factory machines (lathe??) and then adapting what you have got to your problem
Hope you are going to run on 12V car batteries and not on 415V three phase a.c.
The second part of your problem will be how heavy the "tow-car" needs to be. Even if you have enough power, the wheels can slip on the rails and you will go nowhere.