DERAIL, a device used mainly on the sidings of railways to prevent a car that has been moved by wind, gravity or by the error of trainmen from running foul of the main track. It is also frequently used to protect men at work under a car. The derail is variously designed, but generally acts by providing a surface on which the flange of one wheel mounts and crosses over the head of the rail, dragging the wheel at the other end of the axle off its rail. Each pair of wheels follows in turn and, being forced over the irregularities of the road-bed, soon bring the car to a full stop. Some derails are portable, but most are fixed definitely in place and are operated by hand, in unison with a switch or mechanically from a central point. The illustration shows a manual derail with signal target, in operating position.