> Adding capacity to the sky is easy because it's empty
Adding capacity to airports, however, is much harder. Heathrow is at 98% capacity, for example, and it's difficult to add a new airport, or even new runway, for London. See https://en.wikipedia.org/wiki/Expansion_of_Heathrow_Airport , which discusses the major earthworks, etc. involved.
> Roads are easier to maintain and pay for themselves without subsidies just via fuel tax.
In the US fuel taxes only cover about 25% of "State/Local Road Spending", says https://taxfoundation.org/statelocal-road-spending-covered-u... .
> Finally of the few people needed to maintain and run the roads, many are private sector and un-unionized e.g. gas station employees
That's in the US, right? Fuel-tanker drivers in Portugal went on strike last year - https://uk.reuters.com/article/uk-portugal-strike-fuel/portu... .
Fuel-tanker drivers in France went on strike 3 years ago - https://www.thelocal.fr/20170531/french-petrol-stations-begi... .
> It's got nothing to do with greenness which is at any rate a very ambiguous case to make when you consider all the factors. Trains can be less green than planes in many ordinary situations.
Care to elaborate?
For many ordinary situations, train CO2 production is 1/2 or even less than planes, on a per-passenger basis. Quoting https://www.bbc.com/news/science-environment-49349566 "Train virtually always comes out better than plane, often by a lot. A journey from London to Madrid would emit 43kg (95lb) of CO2 per passenger by train, but 118kg by plane (or 265kg if the non-CO2 emissions are included), according to EcoPassenger."
Of course, London-Madrid by train is about 24 hours, or 2.5 hours by flight.
And of course, London-NYC isn't a train trip away, and there are unusual situations like Bergen, Norway—Aberdeen, United Kingdom where there are direct flights while Google Maps just laughs at me for asking for a train routing.
But I'm curious about the ordinary cases where flights are more green than train - that is, where both are reasonable options to consider.