Purchased about 9 months ago and just recently got my mainline running so I could run it. Having an issue with rear truck derailing, just kind of flopping around as though it needs some downward pressure to keep it on the rails. Anyone have a similar problem? The service sheet that came with it (HRBS-475) indicates traction tires on both the front & rear drive wheels. Mine only has the tires on the front....is this correct? Pretty happy with it so far although it doesn't like Atlas crossings very well.