At a particular point approaching a turnout, the front truck of the tender hops completely off the rails and the tender will then lay on it's side. It only happens with one of my Pacifics. I have two. I have loosened the trucks at the front and rear of the tender, to no avail. It does not happen with any of my other 7 steam locomotives or tenders. Is weighting the tender at the front truck the only solution? This is one of the few areas of my layout I do NOT want to rip up and relay.
Check to see if the drawbar hasn't hung up/twisted at either end, coming off an earlier turn or turnout. Compare drawbar play/looseness with the other Pacific.
