A rail track derailer is comprised of a box assembly which includes a pair of parallel, spaced guide plates supported vertically on a horizontal base mounting plate configured for attachment on the gage of a rail to the top sides of a pair of spaced rail ties. The spaced guide plates may be transversely locked to the gage side of a rail. A pivotal arm is slidably attached between the guide plates. The arm includes a derailer shoe along the top of a rail. An arrangement of guide surfaces and followers that connect the box assembly and arm effect positioning of the shoe on and off the rail. The mounting plate may be attached to ties by spikes and to concrete or steel ties by plates on the underside of such toes bolted to the box assembly mounting plate.