I agree with this mostly. Where I disagree is that I don't think this would necessarily dissuade congress from pursuing projects like this if they have extra billions sitting around. I mean even today theres a large group of people (seemingly including the current administration) that basically fantasize over passenger rail in the US despite the fact it would be a complete waste of time and money that wouldn't get used anywhere near enough to justify its existence. Back in the 60s before the complete and total collapse of passenger rail I don't think its unreasonable to say congress would look to invest in some form of expanded network if they had a lot of extra money sitting around. Its rather contingent on who would be in power going forward though. A GOP admin in the 60s TTL would likely just return the surplus to the taxpayers via tax cuts, while a Democratic admin would probably look towards these types of projects and other programs.As soon as the railroads lose the mail contracts for moving mail to the airlines, the fuze is lit for passenger rail collapse in the USA.
Moving mail was the most profitable thing they moved, followed by express freight, then regular freight, and then bulk commodities, with passenger operations losing money except in a few areas, like the NE Corridor.
Mac in place of Ike won't change this.