Marx 4200

I hope this is the correct list to ask maintenance questions about electric trains. I have been on the list for a few days. I am trying to get my loco running. It is from a Marx 4200 set. The engine is a #400. The track and circuit are okay because my other engines run fine. I cleaned and oiled the engine and it started to run. It would only run backwards. It ran good in that direction. Later, it tried to run forward, twice. Made it two times around a small circle test track. Then it wouldn't run at all. The light still burns when advancing the transformer. Today, when removing the loco from the track, I squeezed the sides of the engine (the body is removed), and the loco tried to run, as if there is a short in the wiring. I attempted to find the short. Pressure on the sides made it run once more and now side pressure doesn't make it run again. I tried to find diagrams surfing the net. I don't where the E-unit is. Supposely it could be stuck. The shorting appeared to be in a wire that goes to a spring loaded clip which rest above a felt pad on one side of the engine between the wheels. Any help is appreciated. Thanks, Tom