That is to say, that if we subsidized train travel, then there are benefits (open trade, comms, etc.) that are easily realised but that we don't see.

I feel it is so elastic (sorry, I'm misusing this term somewhat) that you can subsidize to an extremely high percentage without net cost (if all externalities could actually be known and costed in)

I want train journeys to be 50Euro Athens-Stockholm. 10Euro Paris-Hamburg. I want my taxes to pay the difference.