For most parts, you can just wash them in hot soapy water and lightly scrub with a toothbrush. It's important to dry everything off quickly, I typically just set parts in front of a fan to dry. I would only wash parts like that if it's completely dismantled, that way everything can dry properly and you can reach everything. For the motor, I might wash removable parts in water, but not the field or armature. I normally clean the commutator and brushes with cotton swabs soaked in isopropyl alcohol, and use toothpicks to clean out the grooves.
I think this motor should be series. You would reverse the polarity of the brushes or the field to reverse, both would work. I think reversing the field is more common. Unless this locomotive has a reverse switch or reverse unit, it's forward (or reverse, if wired that way) only. I don't know why it would have a rectifier, that motor should run on AC or DC, it's only when trying to run a DC motor on AC is it necessary.
