Rail + Interrail Tickets:

With Trainline it is easy to book train and bus tickets across Europe. They have a best price function and it is possible to select between the connections.

German Train Tickets:

For train tickets in Germany we recommend to check also booking with Bahn.de due to the fact that they also show special price tickets for the federal states.

Baden-Württemberg-Ticket

Interrail:

The Interrail Ticket still exists. Not only for the younger generation but also for adults (60+ reduced) and kids (up to 11 years for free). In the past it was valid for a fixed period to travel on European railways for free. Now it is much more flexible regarding the period and the days within the period which can be used for travel. Have a look on their options. They also provide an app with connections and which shows if connections are completely free or if there are special train surcharges.