As crime is lower in Finland than in Central Europe, theft is very rare on Finnish trains but take care of your personal valuables anyway. The conductor or station official have good English and Swedish language skills. Finnish coaches are comfortable and clean. Seating in 1st class is in compartments. First class is available on long distance trains, but it is not mostly worth the extra money.
All trains have card telephones, mostly in restaurant car.
Now in service between Helsinki and Turku, but in few years they will dramatically cut travel times in Southern Finland. Thanks to their tilt technology and light weight, they can run fast also on old lines. See my Pendolino S220 page . Supplement is obligatory in Pendolino and includes seat reservation. Supplement is in the 2nd class 15 to 35 FIM depending on the length of the journey.