The biggest problem that locomotive has is the 0-4-4 wheel configuration, pretty much a worst case scenario for curves. At least two guide wheels are needed up front, ideally four, to get the locomotive frame started rotating in the direction of a curve before the drive wheels get there. Larger diameter wheels, especially when they are pulling hard and a little sand has been laid on the track, can easily bite into the outside rail and climb over, especially at a joint or worn spot on the rail. This engine could never go around a curve fast enough for center of gravity issues to come into play. Clearly, they never intended to run fast on this line.
None of this prevents the trains from looking incredibly cute, especially in the snow.