What had happened was that the Boers had jammed rocks into the small space between the running rail and the keep rail. When a train takes a curve the slope of the flange allows the inside wheel to slide inwards and the outside wheel to slide outwards. This alters the running radius and helps the wheels to negotiate the curve. The outside wheel flange keeps the wheel from sliding too far but the inside wheel needs a second rail to prevent it sliding right off the running rail. Rocks from the ballast of the roadbed, jammed into the space between the two rails caused the light trucks ahead of the engine to derail as the wheel flanges were lifted clear of the rails. Centrifugal force as the train took the curve at some speed moved the trucks sideways and off the rails. The engine itself was not derailed, its much greater weight crushed the rocks between the rails but the lighter trucks rode up the obstacle. The train was not derailed, as a number of accounts of this incident record, by “large rocks that the Boers had placed on the track”. A large rock on the line would be seen by the guard on the front truck who would have warned the driver to stop. In any case the engine was fitted with a cow catcher which would have cleared the obstruction.