A rotary rail car dumper has a rail car platform forming part of a rigid tipping structure which, through end ring members, is mounted for rotation about a central longitudinal axis for tipping a rail car to dump its contents. Between the rail car platform and the ring members are pin joint connections allowing flexure under gravity loading independently of the ring members, at least before tipping. The maximum stresses occurring during operation of the dumper are thereby reduced. The dumper installation is also provided with means for extending the length of the rail car platform in order to handle rail cars of different lengths in unit trains.