§ 51. Weight of rail. The rail used in the construction or the relaying of the track of every railroad hereafter built or relaid in whole or in part shall be of iron or steel, weighing not less than twenty-five pounds to the lineal yard on narrow-gauge roads, and on all other roads not less than fifty-six pounds to the lineal yard on grades of one hundred and ten feet to the mile or under, and not less than seventy pounds to the lineal yard on grades of over one hundred and ten feet to the mile, except for turnouts, sidings and switches.