Americans used to flying between cities two hundred miles apart find that France often has no such flight. The reason is that high-speed rail has taken those markets outright.
The comparison is door to door, not gate to gate
A flight's advertised duration excludes getting to an airport on the city edge, arriving well ahead of departure, and traveling in from the destination airport.
High-speed trains leave from stations in city centers, and boarding closes only a couple of minutes before departure. The overhead is a fraction of the airport equivalent.
Once that overhead is counted, a train journey of three hours competes with a flight of one hour, and on shorter routes it wins comfortably.
Frequency matters more than speed
A route with departures every half hour is more useful than a faster route with four departures a day, because the average wait for a usable service is far shorter.
Rail on the busiest French corridors runs at high frequency, so a missed train costs a short wait rather than a rebooking.
Airlines cannot match that on domestic sectors without flying aircraft at low load factors, which does not pay.
Rules have removed some routes outright
France has restricted short domestic flights where a rail alternative of a few hours exists, on environmental grounds. The measure affects a small number of city pairs.
Its practical effect is narrower than the headlines suggest, because most of the affected routes had already shrunk under commercial pressure from rail.
Connecting passengers feeding into long-haul flights are treated differently from point-to-point travelers, which limits the disruption to international itineraries.
Geography favors the rail network
The high-speed network radiates from Paris, so journeys to and from the capital are fast while some cross-country trips are slower and less direct.
Flights survive best on exactly those routes, linking regional cities to each other or to Corsica and overseas destinations where rail cannot go.
Reading a French route map by rail lines rather than airline networks generally predicts what will be quick and what will not.
How Americans should book it
A ticket bought as a flight plus a rail segment is often sold as a single through itinerary, with the train leg carrying a flight-style code.
That arrangement matters when a delay threatens a connection, since a through booking places responsibility on the carrier rather than the traveler.
Booking the two separately can be cheaper but leaves any missed connection entirely at the traveler's own cost, which is the trade being made. Rail strikes and weather disruption are the usual reason that trade turns out badly.
Seat reservations on French high-speed services are compulsory rather than optional, so a flexible fare still ties the traveler to a specific departure and changing it means reissuing the ticket.