Eu train companies should (according to EU policies) support one out of two open API standards (gtfs or netex), though they might be difficult to find.

There are a number of projecta that implement these, one of my favorites is https://direkt.bahn.guru/ which shows direct train connections for any (many/most?) European cities with the uae of DBs APIs (sadly it’s currently down until new DB APIs are up again). Github: https://github.com/juliuste/direkt.bahn.guru

There is also the official EU night train map which I frequently use https://back-on-track.eu/night-train-map/

https://omio.com/ is pretty good as well.