Cross Border Rail in the EU is surprisingly bad, IMHO. If you stay within one country, prices are lower, frequency and capacity are higher and usually the speed is higher as well. Eurostar has decreased in capacity over time and is by far the most expensive option, Thalys hasn't increased capacity in decades (bought no new trains) and is also very expensive, ICE services to Netherlands and Belgium aren't cheap and suffer from bad rolling stock (often half of the trains are cancelled), all border connections to spain suck in some form (Madrid-Lissabon is especially bad), there are (almost) no trains through switzerland (to connect to italy) and trains in switzerland are in general pretty slow, making trains to Italy very unattractive. Connections to Denmark suffer from bad rolling stock and are often booked out, there is no way to cross the Baltikum in any reasonable amount of time by train, there are very few train connections left in the balkans after Covid (e.g. greece has no single passenger train crossing the border). For people trying to cross over from Sweden to Finnland over Land, there are trains running to the border towns, but a walk of several kms is required between them (or there may be some form of bus connections, but I can't figure them out). And everything train-wise is very slow in eastern europe anyways. Night trains are very often sold out far in advance and have historic rolling stock with lots of issues.

Worse than that, international tickets are often much more expensive than national ones (compare e.g. Frankfurt-Paris to Munich-Berlin), the border crossing is often the slowest part of the journey and if you want to get the cheap prices on both sides you have to book separate tickets, with all the ensuing fun of figuring out what to do if something goes wrong. Sometimes you just can't book a single ticket anyway. There isn't a great europe wide journey planner (good luck with Lisbon-Madrid or the Euskotren border crossing) with real-time information.

Cross border trains often aren't the priority of a single country (for obvious reasons) and the entity that is supposed to have an eye on eu border travel by rail (the EU) frankly just doesn't care a lot and it shows. The passport controls at internal shengen borders leading to 30+ min delays are just the cherry on top.

Still: A lot of intra-EU flights can be reasonably done by train/long-distance bus and a lot of (especially holiday travel) destinations substituted. It could always be much worse.