Sorry for the German source, I couldn’t find an English one.
Apparently, there are first talks between the German and French traffic ministers to expand the national railway tickets (49€ ticket on the German side, a soon-to-be equivalent in France) in the respecting neighbour country. It’s still a more than early stage, will take several years and will have to overcome the incompetency of Wissing, but the idea is intriguing.
I think the way to do this is to have an open API for booking trains, like planes do. That’s how Expedia, Google Flights, ITA matrix, etc work. https://en.wikipedia.org/wiki/Global_distribution_system
Apparently there is some progress in Sweden about this: https://www.railtech.com/all/2022/11/14/what-can-countries-learn-from-sweden-when-it-comes-to-multi-modal-ticketing/?gdpr=accept
But I think there will probably have to be some sort of EU regulation to get this to happen EU-wide any time soon.