Derailing Device

A derailing device — also known as a derail, or sometimes as a "trap point" in British and Commonwealth usage — is a trackside mechanism designed to intentionally derail a moving rail vehicle that has passed a stop signal or that is moving onto a track it should not be on. The derail throws the wheels off the rail in a controlled manner, sending the vehicle onto the adjacent ballast where it stops by friction, while preventing it from continuing into a protected area where collision or other hazard would be much worse.

There are two main types. A wedge derail is a heavy steel wedge that rests on top of the rail head, with the protected side away from the approaching vehicle. The wedge is positioned so that a wheel rolling over it is lifted up and to the side, dropping off the rail entirely. A split-rail derail (more common in modern installations) actually moves a section of rail aside, leaving a gap that a wheel can drop into and over. Both designs are interlocked into the railway signalling system so that they are "set" in the derailing position by default and can be "removed" only when the signal protecting the route ahead has been cleared.

The contexts in which derails are placed reveal what they exist to prevent. Industrial spurs into chemical plants or oil terminals carry derails at the entry from the main, to prevent a runaway car from rolling into the facility. Sidings leading toward grade crossings or station platforms carry derails to prevent a parked consist from rolling forward against the timetable. Yard tracks adjacent to high-traffic mainlines carry derails to prevent switching mistakes from sending a car onto the main.

For railfans, derails are visible trackside as small steel wedges or movable rail sections, often painted bright yellow with stencilled labels. They are protected by the signalling system from being approached by an unauthorised move, and they are inspected daily as part of the line's routine maintenance. The presence of a derail at a particular location is itself a piece of operational information — it tells you what kind of move would have gone wrong, and what the consequences would have been if no derail were in place.

The deliberate destruction of equipment to prevent worse destruction is one of the foundational engineering trade-offs of railway safety, and the derail is its most direct expression.