In a series of cases the Supreme Court has held that in taxing the rolling stock of railway, sleeping-car and refrigerator companies, a State may estimate the number of cars upon the average kept and used within the State, and for the determination of this average may use any reasonable rule, the one ordinarily employed being that of mileage.41 Conversely that part of the property of a corporation which upon an average is kept and employed outside of the State may not be taxed.42 may also show that its property in other States is of disproportionate value, as, for instance, that it is located in a more densely settled community, where it is disproportionately more productive, or consists of terminals in large cities or other States. All such facts are relevant as bearing upon the value of the State's portion of the entire property. A state statute or a procedure by a State under a statute which denied the company the opportunity of proving such facts, would doubtless be held invalid." Judson, Taxation, § 261.

39 141 U. S. 18; 11 Sup. Ct. Rep. 876; 35 L. ed. 613.

40 198 U. S. 341; 25 Sup. Ct. Rep. 669; 49 L. ed. 1077.

41 Pullman's Palace Car Co. v. Pennsylvania, 141 U. S. 18; 11 Sup. Ct. Rep.

42 Union Refrigerator Transit Co. v. Kentucky, 199 U. S. 194; 26 Sup. Ct. Rep. 36; 50 L. ed. 150; New York v. N. Y. C. & H. R. R. Co., 202 U. S. 584; 26 Sup. Ct. Rep. 714; 50 L. ed. 1155.