So I am not skilled at reverse units, (commonly referred to as e-units since they were initially known as electronic reverse units.) So I will let someone else speak on that portion.
What I will say is the brushes look quite used. That would make me think the commutator would be very dirty.
The brushes are the little round graphite rods held in place with the springs that are soldered in place.
Carefully remove the three screws holding that round looking plastic plate with the wires attached to it. Lift it up and be sure not to loose the brushes in the process. Move it to one side and remove the rotating portion of the motor.
The surface the brushes rub on will look black. Get a red rubber eraser and scrub the gunk off. Do not use anything metal or abrasive.
Put the rotor back in. Make sure the brushes slide nicely(almost never an issue) and reassemble. Try the loco again.
Here are some similar parts from an engine I converted to a can motor. You can see the cover plate, brushes, and motor rotor. The bronze looking plate is the part you clean. A red rubber eraser works fine. If you have a hobby shop nearby some new brushes wouldn't hurt either.
That's my 2 cents for tonight.