Congress ... does possess the power to foster and protect interstate commerce, and to take all measures necessary or appropriate to that end, although intrastate transactions of interstate carriers may be thereby controlled."
Houston, East & West Texas Railway v. United States, 234 U.S. 342 (1914)
The Shreveport case involved regulation of intrastate commerce indirectly because it was done by a registered carrier of interstate commerce. The decision states "by reason of their control of the carriers". What was being regulated was the carrier, as an instrument of interstate commerce. By robertpaulsen's logic, it was necessary and proper for Congress to declare each and every one of us an "instrument of interstate commerce" for the purpose of regulation under the Commerce Clause.