If you buy an Interrail global pass you can travel as much as you like in 32 countries and for 2 days in your country of residence.
https://www.interrail.eu/en/plan-your-trip/trip-ideas/destinations
Don't use the Interrail planner for planning your trip as the timeables in it only is updated once a month. The advice is to use the websites of the national railways for planning.
The advice from the experienced travellers in the community is also to use other ways to make reservations than the Interrail/Eurail website. You can look at the guide in the link:
https://community.eurail.com/train-connections-reservations-47/how-to-get-reservations-105
If you, after having looked at the guide, have questions about how to make specific reservation, please give your travel details (departure date, time and route) preferably in a new post, and you will get advice.
Oh-with some smart planning its even possible to ´visit´=be in 4 or 5 countries in 1 day: like NL-BE-LU-DE-FR (and even CH)
Even though on a map they look like very close together-the old parts of Yugoslavija are much harder, as there are NO trains overborders there. There are buses.