Oscillation.

Oscillation is started by inequalities in the track. It is reduced, or absorbed, by the varying compressions on the springs, which first stop the rocking and then throw it back in the opposite direction. When the spring capacity is insufficient or defective, the oscillation will increase progressively until the wheels on one side are lifted off the rails. Ordinarily such oscillations are dissipated through the springs, but even when the springs are functioning, if there is a succession of cross level variation, the oscillation may become violent. When the load is at the point of maximum oscillation there is a much larger proportion of load on one side than on the other, and at that particular moment the wheels on one side carry much more weight than those on the other. As acceleration varies with a given force inversely as the weight and directly as the time, it follows that when a vehicle is at the point of maximum oscillation (with springs fully compressed and with the vertical reaction through the wheels on one side suddenly increased while being correspondingly decreased on the other) the acceleration of the lighter side is suddenly increased and that of the heavy side is suddenly decreased. The result is that the truck is suddenly and violently slewed, and derailment follows.

This will explain why trucks go off the road on straight track—a fact which has puzzled many railway men. Remember the point of derailment is not necessarily the cause. A track irregularity away back may be (and invariably is) the starting moment of oscillation.