Abstract: A device includes a fixed portion permanently integrated in a platform, a mobile portion connected to the fixed portion so as to be capable of displacement between stowed and expanded configurations, and elements for driving the mobile portion. In order to improve the device safety, including in particularly dangerous cases such as the drift of the railway vehicle along the platform, the mobile portion is provided with telemeters for constant time measuring of the horizontal distance between them and the vehicle, while control members connected to the telemeters process the measurements from the telemeters in order to provide real-time driving-elements actuation signals according to the measurements so that, in the expanded configuration, the horizontal distance between the free end of the mobile part and the access threshold (8A) of the parked vehicle is lower than a predetermined value independently from the relative position of the platform and the parked vehicle.