In American Refrigerator Transit Co. v. Hall, 174 U. S. 70 (1899), the Supreme Court sustained the constitutionality of a Colorado property tax on a stipulated average number of railroad cars that had been located within the territorial limits of Colorado during the tax year, although it was agreed by the parties that the cars "never were run in said State in fixed numbers nor at regular times, nor as a regular part of particular trains." (Id., at 72.)