Trailing "A" locomotive derails

Does that truck turn fully and easily.  Usually this type of derail is because the rotation of the truck is impeded, not because of pressure from the spring.  Can be external from the harness or internal harness that goes around the rear motor in this case.  G

This is my first time with an "A-B" or "A-B-A" set, so ther are a few things to learn.

The rear turck on the trailing "A" unit rotates freely in both directions when held off the track.  The way the wiring harness that forms the tether wraps around the rear motor it would be more likely to impede the truck from rotating in the opposite direction from the one that is causing the issue.

When I first noticed the problem, the tether between the "B" and trailing "A" units seemed too short (it ran along the side of the couplers instead of underneath them).  I thought that it was preventing the truck from centering itself coming out of left-hand turns (pushing the couplers to the outside of the turn).

Late last night, with the shell off, I lengthened the tether again, this time as much as I could without causing a strain on the harness inside as it passses around the rear motor.  I doublechecked to make sure the truck could rotate freely in both directions. I haven't run the locomotive yet to see if this has made any difference. 

It just seems that there is not enough pressure over the two drive axles on this truck to keep it firmly seated on the rails.  The spring pressure on this truck feels the same as the rear truck on the lead "A" unit.  Given the size of the wheel flanges, tt seems like the truck must be tilting toward the axle without flanges (the axle closest to the center of the locomotive), allowing the rear axle on this truck to lift in a turn, pivoting it off the track to the outside of the turn.  When the trainset re-enters a straight section, the truck remains skewed.

I did notice this morning that the tether was centered right over the rear drawbar on the trailing "A" unit, and as such wedges itself between the drawbar and the underside of the carbody when the locomotive is on the track. Perhaps this is impeding the truck from swinging freely, forcing the wheelset to offtrack.  You only notice this when the locomotive is on the track.  With the locomotive on the track I pulled the tether to the side.

It does sound like a tether problem. Try the tether in different positions to see if any work.  The least attractive will probably be the one that doesn't impede the swing of the truck.

.....

Dennis