inadequate, the track may buckle; a large lateral distortion takes place, which trains are unable to negotiate. (In nine years 2000/1 to 2008/9 there were 429 track buckle incidents in Great Britain). Improper operation of control systems Junctions and other changes of routing on railways are generally made by means of points (switches — movable sections capable of changing the onward route of vehicles). In the early days of railways these were moved independently by local staff. Accidents — usually collisions — took place when staff forgot which route the points were set for, or overlooked the approach of a train on a conflicting route. If the points were not correctly set for either route — set in mid-stroke — it is possible for a train passing to derail. The first concentration of levers for signals and points brought together for operation was at Bricklayer's Arms Junction in south-east London in the period 1843–1844. The signal control location (forerunner of the signalbox) was enhanced by the provision of interlocking (preventing a clear signal being set for a route that was not available) in 1856. To prevent the unintended movement of freight vehicles from sidings to running lines, and other analogous improper movements, trap points and derails are provided at the exit from the sidings. In some cases these are provided at the convergence of running lines. It occasionally happens that a driver incorrectly believes they have authority to proceed over the trap points, or that the signaller improperly gives such permission; this results in derailment. The resulting derailment does not always fully protect the other line: a trap point derailment at speed may well result in considerable damage and obstruction, and even a single vehicle may obstruct the clear line. Derailment following collision If a train collides with a massive object, it is clear that derailment of the proper running of vehicle wheels on the track may take place. Although very large obstructions are imagined, it has been known for a cow straying on to the line to derail a passenger train at speed such as occurred in the Polmont rail accident. The most common obstructions encountered are road vehicles at level crossings (grade crossings); malicious persons sometimes place materials on the rails, and in some cases relatively small objects cause a derailment by guiding one wheel over the rail (rather than by gross collision). Derailment has also been brought