> 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.

I have not hear this point before, please would you mind to elaborate, if possible with sources to share? Thanks.

Who subsidizes the air traffic?

Tax payers. In Germany, airplane fuel is not taxed.

See this Greenpeace study (in German) about how much cheaper flying is than train rides and why:

https://www.tagesschau.de/inland/regional/nordrheinwestfalen....

Not taxed does not mean it is subsidized. Subsidy is when money is paid by the government to the airlines.

Tax on fuel reflects the externalities to the whole of society, and arguably it is not high enough.

So exempting tax (equivalently not charging for externalities) is indeed a subsidy, a transfer of wealth from government (and so society) to the airline operators.

Regional airports are usually subsidized by local governments to put them on the map. Also jet fuel could be taxes at à similar rate as gas/diesel