how often do the trains go from helsinki to St petersburg, how long does it take, how much does it cost and and do you have to prebook?
Province: Etela-Suomen Laani
From Helsinki to St Petersburgh there are 4 Allegro trains / day and the price is (one way) 144 euros. There is also 1 connection to Moscow at late afternoon. To St Petersburgh it will take 3hrs36minutes. Reservations and further information vr.fi
Since you will be crossing a national border, I suggest you pre-book your ticket to minimize hassles. Make sure you have a visa which is valid on the day you enter until the day you leave Russia! As Pasi said,
website is the best place to book as it's the website for the Finnish Railways. And don't forget to print out your ticket...!