Abstract: This invention is a cam operated rail clamping device for preventing movement of a structure on a rail comprising a base, a vertically disposed post secured to and projecting upwardly from the base, a tube rotatably and slidably mounted on the post, an air (or hydraulically) operated lift cylinder for raising the tube, a vertically disposed beam extending upwardly from the base and secured thereto, a horizontally disposed flange secured to the lower end of the tube, a clamping jaw carried by the flange and provided with a high point and a plurality of teeth for engagement with a first side of a rail, the flange being mounted for rotation about the post to bring the high point of the jaw into wedging action with the first side of the rail when the flange engages a top portion of the rail, a wearing shoe carried by the base for engagement with a second side of the rail, and a microswitch for indicating the position of the clamping jaw.