The present invention teaches a double-acting compression actuatable slack adjuster to adjust the slack in the brake rigging of a railway vehicle which includes a pair of elongated members movable with respect to each other in a longitudinal direction. A positioning device carrying a pair of opposed abutting surfaces is threadedly engaged with a threaded portion of the elongated rod. Pair of opposite facing abutment surfaces are positioned within the hollow housing. A first urging device is positioned to urge the positioning device into a disengaged position. A second urging device overcomes the force of the first urging device and urges an abutting surface on the positioning device into engagement with an adjacent abutment surface. A first overtravel control device is provided which is activated by a trigger device. A second overtravel control device is provided to prevent jamming condition of the positioning device with one of the abutment surfaces when the elongated rod is fully extended.