A guided vehicle includes a guidance unit having first and second V-mounted guide pulleys resting on first and second running surfaces of a guide rail. The running surfaces are on sides of a median longitudinal plane of the guide rail and each pulley has a flange for freely gripping the guide rail. A device and method protecting against loss of vehicle guidance include a section with longitudinal height fixed along its length to a guide rail part, between running surfaces, the section having a cross-section guiding the unit during derailment or loss of guidance thereof with the section fixed to the guide rail part. The cross-section geometry holds the first pulley flange on the side of the median plane including the first pulley running surface and the second pulley flange on the side including the second pulley running surface. A device fixes the section to the guide rail.