However, airline tend to have agreements in this regard and to have built large networks and alliances for that purpose. (Though imperfect, since some local connections are sometimes hard to get from all airline in the alliance. IIRC TGV+Air is only sold by AF and not DL, despite codesharing on transatlantic flights).

I can see where the single transaction rule might come from in the current EC proposal.

And interactions with Air + Rail passenger rights might get more complex too as a result. (Not to mention Air + Rail is usually a flag carrier with national railway only thing)

And any lesson learnt on how the Rail Passenger directive should be articulated with existing air passenger rights ?

(I was recently abducted by DB while on the LGV Est in France, and sent to Mannheim instead of Saarbrücken, at least I was allowed to take the RE 1 to get home).

In that case, they use a train at least.

"but how were we supposed to know that…"?

By actually using it instead of official cars and airplanes?