If the engine isn't running smoothly at 4-5 scale MPH, it's broken! If it doesn't move at all by the time you get to 3-4 MPH on the remote, I'd be very worried. That is NOT normal! That's the FIRST thing I'd be looking into.
Mine all had to do with the rear truck, I had to put some very small shims in to allow the rear truck pilot wheels to turn freely, and I had to adjust the bend in the truck part with the pilot wheels on it so it would run properly and not jump the tracks. I've run it a few times since, several times for several hours, and so far it's run fine. Oh, it's pulling ten cars, so it's capable of pulling them if it's working properly.
Mine is a PS/3 locomotive, but I bought it used in August of 2013.
