A retracting rail clamp for braking or anchoring a rail-mounted machine such as a crane by clamping opposite sides of a longitudinal rail member, the retracting rail clamp comprising: a load transfer frame rigidly secured to the crane, such as by bolting; and a clamp mechanism suspended within the load transfer frame by at least one lateral pair of pivotally mounted lift levers, and operable through a first and second stage of motion between a raised, brake release position, in which the entirety of the rail clamp is disposed substantially vertically clear of the rail, and a lowered, brake set position wherein at least one opposed facing pair of main levers of the clamp mechanism engage the side surfaces of the rail.