I have a new 80 Ton Three Truck Shay w/Steel Cab #84705. This is the one that came with DCC and Sound. The sounds are wonderful but the running is terrible for a new locomotive. It shimmies in time with the drive. I have two other Shays from a few years back and they run very smoothly. I can't figure out what's causing this side to side shimmy. Have others experienced this motion? Any ideas? It's very annoying to watch.
Turn your Shay upside down, use a couple of clip leads to feed track power to the wheels, and watch them as they turn. Shimmy is often caused by wheels set crooked on their axles, by bent axles, and by out-of-round wheels if the eccentricity is bad enough. If you see any of these, consider sending your Shay back to Bachmann for repair/replacement. It is not a job you can do yourself (unless your hobby room includes a machine shop.)
Jim