Dale says:

The procedures are the same regardless of the make, model, or scale. First, remove the body and examine the drive linkages for something jammed or broken. Next, if you can separate the drive chain from the wheel assembly see if the wheels turn freely. (If this is a steam loco, check that the side rods are not binding.) With the drive linkages disassembled, apply power to the motor to see if it works. If the unit has a smoke generator, disconnect that until the rest of the loco proves working. The problem could be any of the aforementioned things. If all looks good, reassemble the entire chassis, cleaning and lubricating as you go. Test again with power to the motor and repeat the steps above more carefully if it still doesn’t work. If it is okay at this point, replace the body and make sure that there are no places that may be binding on any moving part.

Jerry says:

I’m not sure to what extent you cleaned the engine. Each Lionel engine is labeled by the road number. You can go to the library and take photo copies of the pages in the Greenberg Guide to lionel trains which will have repair instructions. Advice given above by Dale and Frank is a good starting point. If the engine is a steam locomotive disassemble the side arms and apply voltage to the center rail contact and one of the drive wheels. If the wheels still don’t turn, remove the body and apply power directly to the motor. If the wheels still don’t turn separate the motor from the truck and apply power directly to the motor or where the wires connect to the springs that hold the motor brushes in place to see of the motor turns. If the motor doesn’t turn, check the motor brushes to see if worn or not making good contact with the motor armature. You might have to disassemble the motor plate that holds the brushes in place and clean the face of the motor armature with an eraser or alcohol or contact cleaner. Good luck.