These wheels were angled inwards to sit on the load-bearing rails. In addition, each truck had a pair of horizontal spring-mounted gripper wheels which pinched the central upper rail. In non-powered vehicles these were for stability and braking, but in the locomotive these provided the drive in addition. This central rail was not load-bearing. The gripper mechanism allowed trains to manage steep gradients. The rolling stock had the form of horizontal cylinders, including the locomotives, and the edges of the cylinders were rounded. This was an early example of the deliberate design feature of streamlining, which featured in publicity: This system is as applicable for surface as for elevated railroads. It is cheaper to build than an ordinary road, as the design of the rolling stock allows the contour of the ground to be more closely followed. As an elevated road in cities, the permanent structure presents far less obstruction to light and air than the usual form. The center of gravity of the cars and engine is brought down as low as possible, thereby lessening the effect of leverage caused by wind pressure. The smooth, even surface of the exterior of the entire train serve to decrease the resistance to the wind, and permits a high rate of speed. Permanent way The following description is for the expected standard iron construction. As the experimental line demonstrated, much of the construction could have been replaced with wood (timber). The hollow iron support pillars were made up of two C-shaped bars, back to back and bolted together with two strap bars. No specifications were given for the cross-section, "which may be varied as demanded by location", but a standard pillar was suggested at 20 feet (6 metres) comprising 6 feet (1.8 metres) underground on foundations specified for the local geology and 14 feet (4.3 metres) clear, with an additional 4 feet (1.2 metres) occupied by the permanent way at the top. These pillars were to be free-standing, not stayed. The permanent way on which the trains were to run consisted, firstly of a line of single iron structural support girders on the tops of the pillars. On top of these was a line of narrower single girders or track beams (so-called) to which the load-bearing rails were to be fixed. A pair of U-shaped girders, facing upwards, was bolted to the sides of each track beam, and filled with longitudinal baulks