That is not consistent with the arguements presented in the decision. They state explicitly that their jurisdiction is over the actions of the railroad by virtue of their status as a registered carrier.
What you're telling me contradicts the plain language of the decision.
Read the decision again. Checkout where they said Congress “does possess the power to foster and protect interstate commerce, and to take all measures necessary ir appropriate to that end, although intrastate transactions of interstate carriers may thereby be controlled.” There’s your interstate carriers, but there’s also “although.” Seems to me SCOTUS realized interstate carriers could be engaged in intrastate commerce. And such commerce was regulatable not because the carriers were interstate, no, but because they went against the feds’ power to foster and protect interstate commerce. Hence the “substantially related” test.
Here’s the story. The ICC set up a price control regime on interstate rail travel. Certain intrastate lines standing outside the regime “”discriminated” against interstate lines by offering lower prices. This upset the governments power to set prices, so they invented the power to regulate intrastate commerce.
Discriminatory intrastate prices were substantially related to interstate commerce be aide intrastate lines were in competition with interstate lines, not because they were set by interstate carriers. It’s as simple as that.
As for why Wickard did not upturn Shreveport, now that should be clearer. We went from the feds regulating commerce between the states to the feds fostering and protecting interstate commerce, interpreted as fostering and protecting the government’s price controls. From it having to be interstate commerce we went to it having to be commerce that is substantially related to interstate commerce, interpreted as something upsetting interstate price controls.
What was Wickard? An upholding of the AAA, which set marketing quotas and allotments for growing wheat. Which was a means of controlling supply and thereby hopefully buoying prices. A farmer bucked the system by growing his own wheat for his own farm’s consumption. Did SCOTIS misapply Shreveport as precedent in this case because the farmer was not a registered interstate wheat seller? No, because that was not the logic of Shreveport.
The logic was that certain intrastate lines were in competition with interstate commerce. The Wickard decision explicitly argues that the case against Filburn is that homegrown wheat can compete in a sense with commercial wheat. That makes it commerce, and from that point on it tending to upset the interstate wheat price control regime follows the path laid out by Shreveport and others.
So you see it is the fostering principle and the competition principle which unite Wickard and Shreveport. That’s why I say the registered carrier thing is a red herring.