Let's take Rotterdam or Amsterdam as an example. Both of these stations are closed off to non-ticket holders. You can access Dutch trains as well as international trains like Thalys to France and ICE to Germany.
Access is granted by individual tickets bought from the Dutch railway kiosks, by Dutch public transport cards, or by printed/digital QR-code tickets from any of the three rail companies. There is only one type of barrier. There are not isolated entrances for particular lines.
Maybe I misunderstand you, because what you say is close to impossible seems to work just fine in the NL.
But you're right, it's actually not impossible just unlikely to happen. And I must say I really enjoyed the Dutch system where you don't have to print paper every time.
Heh, when we went Amsterdam - Berlin and vice versa we had to print our tickets. We even reserved seats. I quite like that system. One has to reserve in advance if one wants to save costs though.
The downside of the Dutch system is its easy for the government and railroad employees to check where you went. There was even a recent scandal about this, where an API was public which showed the transactions. The price of the transactions could be correlated with the cities the traveller traveled to and from.
In Germany, you have more anonymity thanks to being dependent on paying with cash and paper tickets.
Look, if, with exactly the same cost attached, people don't want that by choice... fair enough. However we don't that choice. And if you travel like 3 times a year a long trip through The Netherlands the 40% discount in down time hours is already worth it. Which is not anonymous but linked to your name. The anonymous card is a joke anyway; not my definition of anonymity that is for sure.
Airports have contracts with each airline flying anyway so they can adapt their systems. Since aviation is a for-profit business (at least in theory), there's more money to do this than for tax funded public transport.
They designed a credit-card sized paper ticket. You can buy a ticket from any station in Britain, to a destination that requires you to change in London (e.g. Brighton), and your ticket will work to travel between the main stations in London. Or you can buy a ticket to a destination in London.
However, this does make some more novel ticket types difficult or impossible. In many other countries, you can buy tickets online and print them at home, or buy rail tickets with a smartphone. These aren't possible in Britain.
A typical barrier in London accepts: National Rail (orange) tickets that used to be labelled "Rail Settlement Plan", Oyster (a contactless travel pass used for most public transit in London), the various contactless payment standards (for your VISA etcetera) and its own pink tickets. But a human is always there if you have something else.