Shannon, in his work on Derailments, disposes most thoroughly of this theory as a cause of derailments. If thought were given to the thousands of times yearly, on most railways, that sudden applications of the brakes are made, without any ill effect resulting, this theory would be ruled out as a causative factor in derailments. But it is extraordinary how widespread is this theory of bunching (always coupled with a sudden application of the brakes), and how persistently it is advanced without a shred of supporting evidence. Shannon says, and says correctly, that when the vacuum brake has been applied throughout the train, if all the brakes are adjusted alike so that each vehicle is retarded by a force proportional to its weight, no vehicle will have any effect on any other in the train. Although such a nice adjustment cannot be expected in practice, yet under working condition when all brakes are on throughout the train, one vehicle will exert but little pull or push on another. When the brake is being applied it acts so quickly that even under the most favourable conditions the speed of the vehicle in front will not be retarded more than two miles per hour before the brake acts on the vehicle behind it. While this difference in speeds will, no doubt, result in a slight shock (if the train is running with loose couplings at the time of the application of the brakes) and slight tendency to derailment on a curve it is most unlikely to be the sole cause of a derailment; more severe shocks are caused by banking engines pushing a train in the rear in ordinary operations or when steam is shut off and an engine is allowed to “drift.”

What Shannon means, in simple words, is that the application of the brake throughout the train is to all intents and purposes instantaneous on each vehicle. Tests on 200 axles have given a complete application throughout in 3 1/2 seconds—this leaves little time for crowding.

The shocks caused by a sudden application of the engine brake must be greater than those caused by an application of the vacuum brake; yet, Shannon says, he could not trace one case in which the Board of Trade officers have ascribed a derailment to this cause. I have also examined many Board of Trade accident returns and cannot find “Brake Application” even mentioned in this connection.

Personally, from a fairly long experience, I am satisfied that the so called “bunching” derailments really result from oscillation brought about by track irregularities and defective springs; this, of course, apart from positive defects in tracks or trucks.