Hi James, Just watched your video'.. Have you tried switching the engines around,, Have the now front trail. They appear to run fine. It looks as though the rear truck or just the coupler arm is not aligned correctly. Is it possible there is a flaw in the coupler arm. Have you examined the bottoms of each loco with a magnifying glass for any irregularities in the frames, coupler arms and trucks. ALso, check and see if all the screws connecting the frame to the shell are fastened completely. One screw incompletely fastened,can create an off balance effect on the tracks'...
I'm curious as well as others are I'm sure. We need to get to the bottom of this issue'...