Having done both the flying and the high speed in a few routes in Europe, the overhead of flying is so much that for anything within about 1000km distance a high speed train is faster, especially because both the origin and the destination train stations are pretty much in the city center and have direct connections to the subway network so you almost always save time and hassle in just getting to the station from your actual origin and from the destination station to your actual destination on the other side, compared to an airport. This is especially so for budget airlines as they tend to use even more peripheric airports.

Further, within the Schengen space in the EU there is no kind of passport or security control, so in most of Europe you literally just walk into a train that happens to go to another country in just the same way as you walk into a train that happens to go to the next city over.

Even for larger distances when the whole trip door-to-door takes an extra hour or two due to doing it by train rather than flying, it’s often worth it because the train is way more comfortable, with plenty of room and you can just get up whenever and go for a walk to the restaurant carriage and get yourself a drink or a light meal.

But yeah, beyond a certain point the train is not worth it anymore. For example Lisbon - Paris by high speed train would be around 6h on a straight line (if you could, which you can’t because of the Golf Of Biscay) at 250 km/h whilst the flight is 3h, but of course, trains in Portugal being shit outside short commuter lines around the 2 major cities and the North-South axis, there isn’t actually a high speed train to Paris or even to anywhere in Spain or even inside the country (fast trains at best, not high speed ones), so you have to take the train and a bus to the nearest point in Spain with a high speed train connection to Madrid and from there to Paris, so you’re lucky if you do the whole trip in 17h.