I have no data to back this up, but I would say train is the most subsidised. The infrastructure required for trains, as well as the maintenance of those tens

I have no data to back this up, but I would say train is the most subsidised. The infrastructure required for trains, as well as the maintenance of those tens thousands of miles of tracks have to dwarf whatever tax break planes get on fuel. Plus airports scale much better than train stations, once it's built you're automatically linked by the most efficient route to everywhere else in the world.