Although I seem to have an opinion on many things, I couldn't even venture a guess about that. I am sure someone will respond within a day or so. What manufacturer, what age loco and which ones specifically?
first you probably need to specifiy what you mean with tip top shape. running a loco for a while will leave marks on the wheels. therefore they're no longer in perfect condition.
A little marvel mystery oil on the gears, q tip with rubbing alcahol cleaning of the wheels, storing them in their boxes when not in use, etc. has worked well for me.
