Technical Help Needed

Well, my father found his old train set out in the garage. We made a nice circular track, put on the locomotive, and flipped the switch. It didn’t work. So he took it to the kitchen table, took it apart, rubbed it with the kind of sand paper you use on metal. It now runs fine. When it’s going backwards. When we put it on forward, it doesn’t go nearly as well. What should be done[?]

I’m always encouraged when someone brings out the old trains… because with them come alot of fond memories! Good for you!
Without knowing much more, Starkat, it sounds to me like the motor brushes will need replacing. Oftentimes, a train that’s run continually in one direction wears the brushes out, and when it’s reversed, as in your case, the brushes are making better contact and the locomotive runs.
If you need to purchase brushes, you’ll need to tell your supplier the engine number. Many parts dealers have online sites.
Be sure the wheels are cleaned with a track cleaning solvent ( I’ve found Goo Gone works well), and give the E unit a quick spray with a contact cleaner (available at Radio Shack).
Make sure you grease and lube the gears and wheel axles, side rods, and other moving parts, and you’ll be ready for years of enjoyment!