score:8
Yes you can. As long as you have a valid ticket for your journey, it doesn't matter where you board the train in. Also, as Lassi Uosukainen stated, Pasila is part of Helsinki and they're essentially treated as the same stop, and tickets usually aren't checked between the two. It's less than a 5 minute distance, so even if they DID check, they couldn't check everyone.
The only "source" I found was this interview in an article regarding another thing: buying a sale train ticket from place A to place B that stops in C, and getting off at station C.
The Finnish Railways former Vice President Commercial Lassi Rönkkönen commented on the matter:
– Se ei ole missään tapauksessa kiellettyä. Asiakkaat hyödyntävät osavälejä näistä tarjouksista, Rönkkönen sanoo Taloussanomille.
rough translation:
It's in no way forbidden. Customers are taking advantage of parts of these campaigns.
Upvote:5
The contract of carriage doesn't mention this, however it states that the passenger shall have a ticket for the duration of the whole trip.
In practice though, Helsinki and Pasila are so close to each other that for long-distance trains they're effectively treated as the same station and I believe no tickets are checked between them. There should be no problem boarding your train in Pasila.