This heavily depends on the type of train you are planning to use.
In general there are 5 different type of trains you can use with your Interrail/Eurail pass
- regional trains which are included in a pass and no reservations are possible
- trains with reservations available but not compulsory. If you don’t buy such a reservation before boarding the train you might not get a seat at all.
- you are not allowed to board trains with compulsory reservation without one. There is mostly only a limited contingent of reservations available for rail pass holders (typically high speed and night trains)
- sometimes you are only granted a discount on standard ticket fares. (E.g. Eurostar and Thalys)
- some trains in Europe such as hotel trains for example don’t offer any discount for rail pass holders
There are the following train categories operating in finland:
- Yöjunat (night train) and Pendolino (highspeed train) require reservations. You can’t take those trains if they are already sold out.
- For intercities (IC) reservations are only recommended. The odds are high that you might not get a seat on this train if it is already booked up.
- Pikajunat (P) and Taajamajuna (H) are interregional and regional trains and do not require reservations. Just board those trains!
You can actually find details on reservations organized by country on interrail.eu