This disclosure relates to a vehicle entrapping or retaining arrangement for a monobeam type of transportation system. The system includes an elongated roadway having a pair of horizontal surfaces straddled by a central H-shaped guide beam. The monobeam vehicles include trucks which have a plurality of supporting wheels which run on the horizontal surfaces of the roadway as well as a plurality of guide wheels which follow vertical sides of the H-shaped central guide beam. A pair of inverted furrows are formed along the upper edge of each of the vertical sides and extend the length of the central guide beam. The inverted furrows are adapted to be engaged by matching lip portions carried on the sides of the trucks so that the monobeam vehicles cannot be overturned by high velocity winds or centrifugal forces experienced in the negotiation of curves and bends in the roadway.