Hi - I’m looking into the 2020 tours and hoping someone can offer advice. Any advice on transportation back to Helsinki after this tour?
If you need to get back to Helsinki from SPB, Allegro is probably the easiest/fastest way.
If you are looking for an extension of the tour, I'd probably go to Moscow for a couple of days and fly out of there.
See details of the train at https://www.vr.fi/cs/vr/en/train-timetables-finland-russia. It's easier and cheaper to buy on that Finnish web site in advance than to buy in Russia.