Abstract: An expanding-contracting device associated with a running linear assembly, and applicable to moving platforms, composed of a succession of forks guided over a looped path, a section of which coincides with a section of the path followed by rods of an endless belt, each fork being adapted to grip a rod and being connected to one of the adjacent forks by means of a connection device controlling, in accordance with a pre-established program, the spacing between the two forks in relation to the position of the first fork, so that the spacing decreases progressively in a first zone of the coincidence section, and increases progressively in a second zone of the section, situated downstream of the first zone.