for a test, put a regular 9 volt inside just to see if it makes it over the switches. Some batteries fail when under load. Take it back out right away.
On some steam, you can add a pick-up to the tender if there isn't one. On diesels, you can add the double rollers when they fit, if they aren't already there.
I've also seen where some wires aren't connected properly or failed, and the engine stalls like you're seeing.