Lionel 397 Coal Loader motor

Up front, I am not a lionel motor expert.

The dark spots on you cleaned armature contacts indicate to me that at least one of the windings is shorted. I believe I am correct in the following:

With armature removed from motor, try:

Number the three armature contact plates 1, 2, 3. Use an ohm meter to measure the resistance from 1 to 2, 1 to 3, and 2 to 3, writing their values down. The values will be low, but should be about the same.

Also, measure resistance from 1 to armature shaft, 2 to shaft, 3 to shaft ==> I believe these three should should open circuit.

Hopefully others will have better advice or more info to help you, but the above should be a good start. Report your results so others get all the info.

Good Luck!!!

One of the brush springs is almost 2 x the other.  That means one may not be applying enough pressure for a good contact.   If the spring is still good and not corroded, it can be stretched longer to match the other.   Careful, too much spring force can add drag. 

I would make sure the ends of the brushes are clean of buildup.   Very fine sandpaper or a Rush Eraser type tool can be used to clean the brush plate. 

With the CB tripping, I would lean toward a short. in one of the windings.   Also check the winding on the field coil.   One end connects to one of the brush ports and the other usually to ground.   You need to disconnect the end from the ground and then do a continuity check between the winding and the casing of the field winding. 

Are there any discolored areas on the windings?   This could point to high heat or a short.   Also smell the windings.  You are looking for the burnt wiring smell.

Rush Earaser