Abstract: In a convertible rail-highway semi-trailer a mechanism for lifting and retaining a rail wheel-set axle unit or bogie in an elevated stored position in the highway mode when the highway wheels engage the ground. The mechanism includes hooks pre-set to engage the bogie when the body has been lowered and to retain the bogie as the body rises when the air springs of the highway wheel-set axle unit are inflated to convert from the rail to highway mode. The mechanism also includes a device to keep the hooks disengaged from the rail bogie when the air springs of the rail bogie are inflated and the flanged railroad wheels engage the tracks and the highway wheels are in an elevated inoperative position. The mechanism is also provided with a valve device to prevent inflation of the rail bogie air springs when the latter is in its elevated stored position.