Placing a washer between the rear truck mount and chassis does not help. The washers just cause the nose of the trailing truck to point downward, they do not raise the chassis. Raising the rear of the chassis will just cause the rear drivers to loose contact with the rails, not a good thing. Looking closely at the above picture the mating joint between the chassis and the motor is visible. The tape is applied to the motor enclosure, the brass flywheel on the back of the motor is visible in the cab.
There are two other possibilities. One is to order scale replacement wheels for the trailing truck. The other way is to buy a set of the AM thin traction bands for the rear drivers. The Pacifics do not have traction tires, AM sells thin bands that can be glued in place on the rear drivers. Adding these traction bands changes the chassis angle on the track, raising the motor almost 1/32" at the trailing truck wheels. I have some of these and test fit them, I think they will also solve the problem. I am fine with the black tape I added.
Here is a picture of an AM heavyweight three axle truck. The center wheels are scale, the outer wheels are high rail. The reduced height of the scale flange should work.
