Lionel transformers were built to last. Perhaps the most frequent issue with them is a messed-up power cord. But sometimes the cord is OK but it needs a new plug.
Replacing a power cord is easier than it sounds, but replacing just the plug is easier still.
First, make sure the cord itself is still in reasonable shape. It should be flexible, not brittle, with no breaks in the insulation. If the insulation is breaking apart, you need to replace the entire cord. Next, conduct as much of my transformer safety check as you can without plugging the transformer in. Repairing a transformer that doesn’t pass those tests needs to be done by a professional.
But if the transformer passes all of those tests except for having a damaged or missing plug, you can get by with just replacing the plug.
To replace it, all you need is a set of wire strippers/cutters, a screwdriver, and a replacement power plug, which won’t cost you much more than a couple of dollars.
If the plug is missing altogether, then someone already started the work for you. If not, cut off the damaged plug with a pair of wire cutters. Strip off about a half-inch of insulation from both wires, then twist the copper strands together nice and tight.
Disassemble the replacement plug, then thread the two wires through the hole in the plug. There are two screws inside. Bend each wire into a U shape, and loop one wire over one of the screws and the other wire over the other. Cinch the U shape closed, then tighten the screws. Slide the protective plate over the end of the plug. And that’s it. Now you can conduct a further safety check to ensure the transformer is safe to use, then enjoy years of further service.