The route you're talking about is basically Rail Baltica (https://en.wikipedia.org/wiki/Rail_Baltica) and the Helsinki-Tallinn Tunnel. The first project is agreed upon in theory, and the latter one is also included on long-range plans, although there is some criticism of the project on grounds such as financial feasibility and other issues (like the tracks being the only 1435mm track in these countries, who normally use 1520mm (or 1524mm) gauge).

Wow, that looks like a great route. I had to check it on a Map to see what shape it would make. It has a nice curve to it :)

https://www.google.com/maps/dir/Helsinki,+Finland/Tallinn,+E...

I'm travelling from The Netherlands to Berlin next week and although the train is comfortable. It's a bit annoying that it has so many stops in between. (taking the DB train, not the high speed one).

East of Poznan? I leave east of Poznan and the roads are good in winter, even when we have more snow than the recent winters.

Most drivers know how to behave in such conditions and drive more slowly (and have winter tires). And roads are mostly kept in good condition (if you allow 1-2 day after first heavy snow during winter).

Not sure how the conditions are in other countries during the winter.

Very easy safe route, leaving lunchtime on Tuesday from Amsterdam will get you the ferry from Travemünde Tuesday evening (depart 3AM wed morning), arriving 0900 Thursday morning in Helsinki