In Adams Express Co. v. Ohio, 165 U.S. 194 (1897), the United States Supreme Court noted:
As to railroad, telegraph and sleeping car companies, engaged in interstate commerce, it has often been held by this court that their property, in the several States through which their lines or business extended, might be valued as a unit for the purposes of taxation, taking into consideration the uses to which it was put and all the elements making up aggregate value, and that a proportion of the whole fairly and properly ascertained might be taxed by the particular State . . . .
The valuation was, thus, not confined to the wires, poles and instruments of the telegraph company . . . but included the proportionate part of the value resulting from the combination of the means by which the business was carried on, a value existing to an appreciable extent throughout the entire domain of operation.
(Adams Express, 165 U.S. at 220-21.)