Dad's Dilemma
Dad's Dilemma
For starters, you have just given the best reason why the first train set should be new.
In dealing with a newly acquired used engine four things come to mind. the age and use of the engine, does it show evidence of being Fixed, electrical and mechanical problems.
The engine could just be worn out. The armature surfaces are scoured, brushes are worn. Springs need tension on the brushes to make contact. Sometimes they gum up with oil.
Previous work, I find a lot of axles reversed on tenders causing a short. From your description it could be just that. Look for, bent axle, binding on the drive wheels from the frame or gears filled with hair etc. Weight is necessary to drive the wheels down for contact. I have tested engines without it an they barely run.Also some engines have rubber drive wheels if these are missing you will see a groove in the wheel.
Electrically your track is clean. It would help if you had a light across the rails. Then if you set the engine down the light would die indicating a short. Now ,the wheels need to be clean, the wiper contacts for the wheels need to be clean and have tension. The wire may have a bad contact under a stress point. The last thing, you can do is run the engine directly from the transformer. Being old, it may just need a good run. If it squeeks, oil lightly.
Look for wobbily wheels. See, if the motor easily responds to power changes. Lastly, touch the motor to determine how much heat is generated.
Does it show that I worked on a few oldies?

If you have a can motor you can't see anything, but the gear at the armature often slips. This gear handles a lot of torque. If it slips, that's all folks.

hwell:
Let us know the prognosis.
If all else fails ,buy an engine of the same type and manufacturer and switch bodies. LOL