Schedule reliability

DB don't operate this train. In such a case, only rely on what the operator tells you.

You didn't mention why you need to be in Hamburg by 22:00, but if it's to catch the 22:11 night train to Basel and Zurich, then I hope you have a plan B in case the connection fails.

You are right in both points, but first, the train Operator (Snalltaget) did not answer or their website did not work (I could not open the reservation, though I had the correct details and e-mail address); and the other point, to get to Hamburg in time, well, yes, but it costs 170 euros...

​@Olli Heinonen If i remember correctly you can cancel Flixbus tickets for free if its more then 30 days until the planned travel date. (If you dont plan to take the bus anymore)

Yes, that’s correct, but you don’t get the refund as money, only as a voucher.

“The only lesson I’ve learned from this is that you can’t rely on anything”

I think the lesson is that (especially for particularly tight or complex connections) you need to verify with the company that operates the train if you want to get as close to the truth as possible.

As mentioned above, Snalltaget run the particular train and they would be the official source of the timing information.  While other sources can normally be relied upon, it’s unfortunate that you came across one of the scenarios where this didn’t hold true.

Please don’t take it as representative of the Interrail experience because it’s very far from the case.

I'd also think you don't need to open your reservation to verify Snälltåget's timetable, since you can just look up your train on their booking page.

If I understand your itinerary correctly, you planned to set off from Helsinki on the evening of day 1, sail to Stockholm (arriving on the morning of day 2) and then take the direct train IC307 to Hamburg, arriving around 21:57 on day 2 . . . and immediately connect to a sleeper train departing at 22:11.

While it’s admirable planning and efficient use of pass time, it all falls down if (when) the train arriving into Hamburg gets held up for anything at all - even just a problem with another train which keeps your platform occupied.  The night train could be rescheduled to start a little earlier because of any number of things, or it could start at Hamburg Harburg.

Basically, I think many people would advise against planning such a tight connection between two trains which both run only once a day.  One glitch and you are stuck a long way from where you need to be.

Again, I think it’s an admirable plan and possibly even something I might try myself, but not if I absolutely had to be somewhere for a specific date . . . and get back to where I started all within a few days.