> This is because of the extreme subsidies air traffic receives.

I don't think so. Railway lines are the epitome of lost fund subsidies by the state. You need to build and maintain a new line for each direct connection you want to create. The construction cost of a railway line, let alone a line that supports high-speed rail, eclipses the cost of a brand new airport.

Once a railway line is in place, getting a train to travel from point A to B costs peanuts.