Hi Mike,
Sounds like you're after an in-service checklist, not a maintenance list.
The in-service stuff has been pretty well covered, but here's some general maintenance stuff to look at. Some of it depends on how and where you store your equipment...
Loose detail parts, some things mysteriously detach themselves after being used for a while.
Condition of paint, touch up derailment damage if needed.
Wheel journal/bearing wear. Maintain as needed, if they're sealed like mine, RTF.
Wheel buildup/wear. A good time to scrape the buildup off of the wheel treads if there's any. If you put on many miles, make sure they are not dished.
Make sure the screw holding the truck to the car isn't loose or working it's way out. If it's already fallen out, you know it since you're holding the car. My freight cars rarely get fingered and I occasionally find one where the carbody has been riding around on the truck by gravity for who knows how long.
Couplers, make sure they haven't started sagging for some reason. Also, make sure one (or both) springs haven't gotten weak or somehow managed to escape the coupler pocket.
That's the normal stuff I check, YMMV.
Regards,
pk