We have a GS4 Daylight 4-8-4 (# 11302) and the last set of driving wheels (rear ones) keep on uncoupling from each other. A small piece of plastic the shape of a half shaft has fallen off and its obvious something has broken inside. We tried to open the base of the locomotive but to no avail as the front screw (by the leading bogie/pilot truck) is so tight we can't unlock it.

Is this a common occurrence and how can this be fixed? Where/how do we get replacement parts?   

We're no experts on this subject and could use some advice on fixing this problem by ourselves if possible. If not, please let us know if we should send this for repairs at Bachmann.

Many Thanks for your feedback