What a wonderful trip you have planned. I hear the passenger trains travelling this route are amazing! Remember to take a face covering with you - many countries require that if you are travelling by train these days. If you wish to visit all of these cities, then I would suggest the following route:
St Petersburg > Helsinki > Stockholm > Oslo > London > Amsterdam > Cologne > Paris > Bordeaux > Zurich > Milan > Marseille > Barcelona > Rome > Sofia > Athens > Budapest > Vienna > Munich > Prague > Warsaw > Berlin > Hamburg > Gdansk > St Petersburg.
Of course, the joy of this route is that one could start from any point. I would love to visit these cities too, so let me know how the trip goes and I might try it for myself!
Have a great time and don't forget to post some pics of your trip,
(COM202 Loch Ness)