A lateral extension (1), for road and rail vehicles, comprising a multitude of individual shafts (5) assembled so as to pivot freely, at each of their extremities, firstly, with regard to the interior extremity secured to the vehicle, on a longitudinal structure (11) that is fixed in relation to the load-carrying platform and, secondly, as regards the exterior extremity, on an edge and connecting rigid longitudinal member (10). The shafts bear, over a portion of their length, on an adjacent flat support surface (13) that is fixed in relation to the load-carrying platform, and are cantilevered towards the exterior over the remainder of their length. The extension can be moved to a deployed position, so that the supporting surface (14) provides a passageway, and, a retracted position of lesser width, and incorporates a railing. A changeover from one position to the other is accomplished by pivoting in a horizontal plane.