I recently got a used MTH LIRR RS1 30-20247-1, and it has an issue with the back truck I cant figure out. When the engine is running forward (long hood forward), it makes a sound and vibrates as if gears aren't right. It almost sounds and feels like when you run a postwar Lionel engine at slow speed and you can feel and hear each tick of the gears. it also get more noticeable/louder when the engine is pulling something
I took apart the entire truck/motor assembly and could find nothing wrong. I disconnected the motor and the truck rolled smoothly and freely and the gears all spun. I put the whole thing back together and I still have the same issue. It's well lubricated, in fact, maybe even a little too well lubricated from the previous owner. I also checked and I couldn't find any wires or anything that could getting caught in the gears.
I have 2 other MTH RS1s that run like swiss watches. It's also worth noting this issue doesn't seem to effect being able to run at slow speed or pull, it can do both those things, but its noticeably noisy and if you put your hand on the back pilot, you can feel it vibrating.
Any suggestions on to what it might be?