Abstract: A railcar door assembly includes a drive mechanism operably connected to elongated support members for selectively imparting rotation of movement thereto, and a retarder assembly for preventing movement of the drive mechanism beyond defined limits. The drive mechanism includes a gear segment mounted on a bearing plate, and the retarder assembly is comprised of a pin and a retarder block. The pin extends through openings in each of the retarder block and bearing plate. The gear segment is rotated until a lobe of the gear first contacts and then compresses the block, thereby preventing further rotation of the gear segment. This arrangement effectively prevents undesired drifting of a railcar door into the side of the railcar itself.