Well, changing traction tires on newer MTH diesels is very easy, remove the side frames with the two easily accessible screws from the bottom, you have full access to the wheels.
As for changing lights, you don't do that. All the lighting on PS/3 is LED, and they're supposed to last the life of the locomotive, same with the supercaps that replaced the batteries.
Now, they should have something on maintaining smoke units, those are arguably the most maintenance intensive items on most locomotives. 
