There are in effect two separate ticketing systems, one for the long distance trains run by VR and one for the Helsinki Commuter Region. You *can* buy tickets for VR within the HSL area but if you're caught without the correct ticket it can get expensive. Given the expense of doing this however I have no idea why.
If you want to experience a loco hauled train then I would recommend you take a trip to Turku, Tampere or Lahti (via the high speed line) - the latter takes about 45 minutes from Helsinki.
Tickets are most easily bought using the HSL app which covers a huge area (Espoo, Vantaa, Kauniainen etc) or through the VR app.
Any questions just PM me - I live there.
t.
Moi RailExplorer - missä päin Helsingissä? Lähetä PM minulle jos haluat!