MTH Dorfan 1134 Won't Run

Little update, I took the shell off (which includes the speaker & smoke unit) and the motor runs with it detached. As soon as I put it back on it doesn't run, disassemble it again it runs. Maybe I'll try and disconnect the individual components in the shell to see if that affects it.

So, it ran with the speaker disconnected as I held up the shell. I reconnected the speaker, it still ran. It runs with me holding the shell over the train as it moves along the tracks and with all the components in the shell operating. Is this most likely a grounding issue to the shell?

that's what it sounds like (and I have 0 electronic knowledge.) I had a Postwar steamer that would only run with the shell off. I couldn't find anything, so a large piece of duct tape on the inside of the shell did the trick.

I found that the connectors to the motor were loose, every time I put the shell on it would jiggle them enough to make it either not work or work intermittently. I crimped them and it seems to be working ok.