In Atchison, Topeka & Santa Fe Ry. Co. v. Vosburg, 238 U.S. 56 (1915), a statute requiring prompt furnishing of cars by carriers, and prompt loading by shippers, and which imposed the same penalty per car upon delinquents of either group, but which added attorney's fee to the penalty imposed on the carriers in case of recovery by a shipper, was held to deny to the carriers the equal protection of the laws, because in such a case there was no ground for putting the carriers in a different class from the shippers and imposing a special burden on them, when they were both in identically the same situation.