1) Dismantle the locomotive and remove the trucks.
2) Place the trucks in a shallow Tupperware container half full of mineral spirits (odorless spirits). Allow to soak for an hour or two to loosen dirt and oxidation.
3) With a nail brush or firm toothbrush, gently scrub the trucks and couplers.
4) Use a dental pick or other similar implement to remove any remaining oxidation.
5) Lubricate and reassemble.
Good luck!
Mitch