Not science fiction, you guys. The Kremlin actually contacted Japan about this as part of Putin's initiative to foster cooperation between the two countries. The idea is to extend the current Trans-Siberian line all the way to Hokkaido, an island located at the far northern tip of Japan. Guests can take the train all the way to Tokyo. Reversing course, they could also ride all the way to Moscow, the European end of the line, and take a train to London. WHAM BAM THANK YOU MA'AM!
A flight from Tokyo to London is 12 hours and this proposed ~6K mile train ride could take up to a week. Long and expensive but one can go through seven diff countries.
Sources:
http://english.hani.co.kr/arti/english_edition/e_international/764064.html
https://omgfacts.com/this-railway-could-link-london-to-tokyo-a7dfbc1c59e7#.p5059jxs6