 In his patent of 1846 he describes his 'Aerial Wheels' as being equally suitable for, "the ground or rail or track on which they run".The patent also included a drawing of such a railway, with the weight carried by pneumatic main wheels running on a flat board track and guidance provided by small horizontal steel wheels running on the sides of a central vertical guide rail.During the World War II German occupation of Paris, the Metro system was used to capacity, with relatively little maintenance performed.