In a rail spike driving machine having a frame, wheels and a spike driver gun slidable in parallel relationship to the rail, and control means for limiting that sliding action, an apparatus for releasably locking the position of said gun above a particular tie comprising a telescoping tube with an inner member and a hollow outer member, the inner member inserted at one end into the outer member and at the other end to the spike driver gun, the outer member receiving the inner member at one end and slidably secured to the frame at the other end to control the perpendicular action of the spike driver gun in relation to the rail, and a releasable brake assembly mounted on the outer member so as to exert a locking force on the inner member and prevent lateral movement of the spiker gun.