Huzzah, huzzah!! After much taking apart and poking around my Rivarossi Big Boy is running beautifully. shaygetz got it right with his guess of a rivet catching a rod. But the cause was in the front truck/engine, not in the rear truck where the problem was occurring. See my original thread here [thread]19145[/thread][thread=19145][/thread]. After my buddy and I took the Boy apart we did find that a rear axel was too fat for its flange, but the engine still didn't run even after filing the axel down. I took the engine to a place in the Milwaukee area that does a super job of fixing model trains. Now it runs like a dream! One problem though: in all the back-and-forth from my place to my friend's to the train shop, the right-side cab handrail disappeared. Could someone here who has a Big Boy post a photo of the cab, showing the handrails? I'll have to fashion a new one from brass wire. Muchas gracias for all the help!