I just wrote an entry about the easy repair and motor upgrade for a diesel F7, but other repairs are not so easy. If general maintenance is all the doctor ordered stay away from tear downs of locos with side rods except if you have nothing to lose due to a major issue, wheels are completely tangled in who knows what in this case the choice is easy but not the repair. I find repairing locos to be very enjoyable, I have repaired every type of z train including complete tear-downs. Taking everything apart and putting back together again is easier than Humpty Dumpty by a long shot, but aligning side rods can be difficult work for the uninitiated. Side rods on locos attach to multiple wheelsets, their function on models is decorative, but if they are poorly aligned they will not move, or if pairs are misaligned directional travel is interrupted such is the case with the Swiss Krokodil: loco might only run well in one direction if the side rods are not synchronous.

Word of caution: get some experience with diesels and electrics before moving into steam engine repair.

Particularly difficult locos to tear down are all versions of the ‘mallet’ locos, followed by Swiss Krokodil, and finally followed by big steam and little steam.