Hi, I just phone called Silja Line to get some real info on their current Interrail discounts between Stockholm, Sweden and Turku, Finland.
The girl on the telephone said that they sell deck places (no cabin required) between the day ships between Stockholm and Turku. For the night ships between Sweden and Finland, a cabin is required.
I also found out that you can get the Interrail cardholder price by entering 'EURAIL' to the promotion code box on the web reservation form. I've included price examples below.
With Silja Line the prices are for the whole cabin, not per person (except for the deck places).
So when for instance you buy a "C" class cabin, the price is the same whether 2, 3 or 4 people travel. If you travel by yourself, the good choice is cabin class "B2" for 1-2 persons. "C" cabins are furnished like "B" cabins, but they are on the low deck (2) so there might be some engine noise, though it hasn't bothered me. "A" class cabins have an outside window.
Here are the prices I happened to get just now from the online reservation system:
Prices for a Morning ferry from Stockholm to Turku
or from Turku to Stockholm:
Deck place: 16 EUR per person
A class: 78 EUR per cabin (contains the ticket price for 1-4 people)
B class: 59 EUR per cabin (2-4 people)
B2 class: 53 EUR per cabin (1-2 people)
C class: 51 EUR per cabin (1-4 people)
Timetables (all times local, Sweden:CET / Finland:EET) :
depart TURKU 08:00 - arrive STOCKHOLM 18:15
depart STOCKHOLM 07:10 - arrive TURKU 19:15
Prices for an evening ferry from Turku to Stockholm
or from Stockholm to Turku:
A:117 eur, B:88 eur, B2:79 eur, C: 75 eur for the whole cabin.
depart STOCKHOLM 19:15 - arrive TURKU 07:00
depart TURKU 20:15 - arrive STOCKHOLM 06:10
At the Turku Harbour, the railroad is built all the way to the ferry terminal.
As far as I know, using the ferry does not "consume" your travel days on the pass.
You can do the reservations at the website www dot tallinksilja dot com slash en.
Try to use the aforementioned promotional code in case it still works.