The main part of the wheel tread is an  inward inclination of 1:40 .  In order to make the top of the rail track under the wheel load with cone tread stressed uniformly ,decrease load offset, reduce rail web stress , avoid slit between rail head and rail web, the railway track shall not be laid upright for straight line, it shall be inward inclined ,this inward inclination of rail called Rail Cant, it is a high precision ,portable , numeric rail geometrical testing tool.