For toy train motor overhauling I've settled for the CRC Lectra-Motive. I tried disassembly, then hand cleaning with alcohol or similar, it always was a mess and left my hands black. Now I just spray the CRC on everything, gears, armature, contacts. The gunk that comes out of some of my pre-war motor projects is pretty amazing. Always makes starting the tear down job a lot cleaner.
Among my other crazy hobbies, antique cars, hi-fi equipment and machine tool rebuilding, if I specifically need a contact cleaner, I go for the Caig DeOxit. I've rescued rheostats, industrial motor contacts, automotive switches, etc with that stuff.