On one travel day you can take as many trains that you can catch. 

If you board a train before midnight, it counts as 1 travel day till exit of the train. For Example with an nightrain you get on board at 20:00 at get off the train (without changing the train) at 9:00 on the next morning. This counts as 1 travel day. If you take AFTER midnight a new train, than you need a new travel day. 

Please note, that some trains need a reservation and that only on 2 travel days you are allowed to use it in your country of residence (Outbound/Inbound rule) 

´some´: be warned, in FRance, ITaly and even more in ESpana about ALL-certainly the fast and long-distance trains MUST be reserved (=mandatory), this also holds for nearly all overnight trains (with sleepers etc.). Every day this forum is full of people complaining and asking for help for this. You can go there of course, but be aware of this situation!

On the other hand-in the Balkans, also a favorite for many, there are hardly any trains over the border left and IN some countries very, very few. You cannot reach GReece without suing some other transportation as trains.