you see a worm bearing fly out from under the body one way & the truck try to go the opposite direction.
After a few choice words, remove the coupler boxes, remove the body, return the part that tried to get away back on top of the truck, put it all back together & relaxation commences again.
The culprit is the loco on the right, pic was taken years ago on the old layout @ the old house.
You are fortunate that your little piece did not take a trip down an air duct or get batted into some inaccessable spot by a pouncing feline.
Have fun,
Richard
If not a cat, how about a dog who thinks that flying morsel must taste wonderful, and it’s gone in an instant. SLURP!!!
Or fall into the time/space warp that is about an inch off the floor… Blink!
Those always turn up in the end…if you look carefully 
It looks like you’re describing an Athearn SD40. I’ve taken a few of those apart and put them back together. The Life-Like S-3 (coupled to the gondola) can be tricky to take apart too.