If it's a night train, it may be a good idea to have a reservation. You can get an idea of the number of free seats when you start a normal booking on www.vr.fi and use the seat selection.

If you're thinking about booking via Interrail: don't. For Finnish reservations, they only have the option of delivery by post and that currently takes an awful long time and costs an extra 9€ per order (and it is on top of the 2€ booking fee per person and train). Instead, book at a Finnish station or call VR (scroll down to Finland).

By the way, there are also sleeping compartments in these night trains. Book them via VR (station or by phone).

Was in FI recently myself and was told that NO RES are needed for Passes. BUT as all local tix have included RES-and seats are not marked-it means that @ every stop someone can come and send you away. It may even happen that the seat you are on is ´sold´ to a Finn only later from a stop onward-this will of course not show on a seatplan. There have been several discussions about it and thus I strongly advise you to try again-VR only has now a very few staffed counters-and these have very limited hours-as about all normal things are done via web and fone there. Sleepers are of course more comfy-but I doubt there are still free a day or 2 ahead in busy hi-season

The trains as such are really very good and comfy there

Sure it will, unless it is a new reservation, sold after you looked at the seat plan.