What if the state does decide to subsidize the rail line, but the shippers continue to use trucks and barges, because the mere existance of the rail line helps hold barge/truck rail rates in line whther they use rail or not?

Unless there is some ironclad mechanism for making certain that the shippers actually use the line to haul freight rather than as a club to keep their truck/barge costs down (1930's rate levels, indeed; we should all be so lucky as to be paying prices set 75 years ago!), you'll end up with the worst of both worlds: A heavy state subsidy for the unused rail line AND the heavy costs of road upkeep.

You'll have happy shippers, though!