(2-part post):
Rail times throughout Europe are available on the German Railways website:
http://reiseauskunft.bahn.de/bin/query.exe/en
I've just asked this site how to get from St Petersburg to London, starting out on Monday, January 30th. Here's one of the routes:
Depart St Petersburg 1750 Monday
Arrive Orscha 0424 Tuesday
Depart Orscha 0534 Tuesday
Arrive Brest 1250 Tuesday
Depart Brest 1320 Tuesday
Arrive Cologne 0617 Wednesday
Depart Cologne 0714 Wednesday
Arrive Brussels 0932 Wednesday
Depart Brussels 0956 Wednesday
Arrive London 1126 Wednesday
Total journey time = 44 hours 36 minutes.
(NB: In practice, I don't think that there's enough time for Eurostar check-in allowed in that schedule - so you should add another hour or so on to allow for catching the next Eurostar service).