Abstract: A position locking feature for an expandable trailer by which the trailer may be automatically locked in either an expanded or a retracted position. A stop block having a transverse striking plate is mounted to an expansion box beam carrying the expandable portion of the trailer. A locking assembly is mounted to a fixed box beam carrying the fixed portion of the trailer. The locking assembly biases a plunger toward the expansion box beam such that the plunger abuts the transverse striking plate, thereby preventing relative movement of the fixed and expandable portions in one direction. A pair of such stop blocks may be used to lock the trailer in either of the expanded or retracted positions.