For anyone who has a long stopover in Helsinki, I would recommend to buy the Day pass for Zones A, B, C this includes unlimited rides between these zones for the day, which includes the airport to City Center train, which a single fare is 4.10 EUR, and the day pass was 11 EUR.
That also includes the Ferry from Helsinki to the Suomenlinna island which is on the UNESCO list. When I wanted to go there, I was told I need a minimum of 3 hrs to explore the island, and I had to catch the ferry to Estonia, so I was afraid I will miss it. I would say, if you go straight from the Airport to the the City Center and from there you take a Tram, than the way from the Airport to the Ferry terminal is about an hour, I would give for the ferry going and back plus approximately 4 hrs to explore, that would be about 5 hrs. An hour to get back to the airport.
So if you have a stop of 9 hrs you will be good. At your own risk...
The Helsinki Transport didn't accept Amex, it was a hassle for me, plus the machine doesn't give change, so I had to run back to the airport to break a 20 EUR bill to have exact change.
