James Madison based his constitutionally required veto letter for the public works bill on a discussion about canals and roads at the Constitutional Convention. Benjamin Franklin had suggested adding canals (for moving freight) to Clause 7 of Section 8, the clause that gives Congress control of mail roads. But the delegates had rejected the idea as evidenced by the following excerpt from Thomas Jeffersons writings.
A proposition was made to them to authorize Congress to open canals [emphasis added], and an amendatory one to empower them to incorporate. But the whole was rejected, and one of the reasons for rejection urged in debate was, that then they would have a power to erect a bank, which would render the great cities, where there were prejudices and jealousies on the subject, adverse to the reception of the Constitution. Jeffersons Opinion on the Constitutionality of a National Bank : 1791.
Regarding constitutionally defensible federal spending projects, given FDRs popularity he could have led Congress to propose amendments to the Constitution to the states before establishing his spending programs, the states probably ratifying them. Instead, he knocked the Constitution off of its foundations by leading Congress to establish his spending programs without the required consent of the Constitutions Article V state majority.
Same issue with Eisenhower. He should have taken advantage of his popularity to establish the nations interstate highway by first getting the states to appropriately amend the Constitution.
I’m afraid I’m not following your logic. A canal is not a post road insomuch that it is not even a road.
As for the power of the bank I dont even see how that is related to the subject whatsoever.
A owning and running a bank is not after all required for or really even related to a post road.