Hey,

you best check out the local railway operators websites/schedules. Railplanner App doesn’t include this timetables.

Cross-border services are very limited → best to take a bus in some cases to cross from one country to another.

Ok, thank you for the reply ! 

But does it mean I have to pay extra ? Or is it included in the Global Pass nonetheless? 

For the buses you have to pay extra. E.g. luxexpress is operating buses there. They have also a student discount I think (10%?). In general Interrail doesn’t really pay off for the Baltics. Only pays off if you continue to Finland or if you have a long-term Interrail and just spend some days in the Baltics. Otherwise single tickets for the train and bus (which is so or so not included as said) might be the better option.