After some chatting with a Bachmann representative...

Apparently, the older 4-4-0 drive shafts had a square end and the newer ones have a arrow-like head. I suppose I'll have to look around and see if a third party makes a drive shaft. Hmm...

I have a Bachmann "Old West" Jupiter 4-4-0 locomotive. In these locomotives, the motor is in the coal car and has a small "drive piece" that runs between  the coal car and the locomotive to turn the gears in the locomotive to make the locomotive run. I had picked up the entire old set at a yard sale for $5 and this is the last repair I need to do to get the train running. Does anyone know where I might be able to order a replacement part for this train so that I can save it? Otherwise, it might just have to sit in my roundhouse as a display piece when I eventually buy one. Thank you.