Apparently the fact that every mile driven on American roads and every ticket bought on airlines are also subsidized indirectly seem to not occur to these nimrods.
Over the years, I have stayed out of this argument when it has been brought up here, but now I will get into it. And I'm sure I will not be popular.
The Constitution expressly gives Congress the power to build intercity highways, or as they were called then, "post roads." (Article I, Section 8.) Therefore, spending money to build and maintain intercity highways is constitutional. The Constitution does not give Congress the power to operate transportation companies. Therefore spending money to operate a railroad is unconstitutional. (Amendment X.)
Railroads and airplanes did not exist when the Constitution was written, or course, but coaches and ships did, and the Constitution does not give Congress the power to operate stagecoach lines and merchant maritime fleets.
What were the Framers trying to accomplish by granting Congress the power to build intercity roads? They were trying to build an infrastructure that would knit the states together and allow commerce and communication among them. (It is worth noting that intrastate infrastructure projects like the Erie Canal came to the Federal government for funding and were told to get lost, because Congress could not spend money on things that only benefitted one state.) If they were writing the Constitution at a time when railroads and airplanes existed, the Framers might have included things like airports and intercity railroad rights-of-way as things Congress could build, because such things are nodes and links of a network, and building such a network helps tie the country together. But they would not have given Congress the power to operate railroads and airlines.
Remember, when in the 1860s the US government wanted a transcontinental railroad to tie the new Pacific coast states to the rest of the Union, they did not create a federal railroad -- they did not have the power, so that would have been illegal -- but instead paid companies to build it and then left those companies to operate it.
It is no coincidence that the unconstitutional Amtrak was created by the most "progressive" president of the last 50 years, Richard Nixon.