This is in post-1900 because I'm not sure when the Russians moved it.... but what would the effects down the road be if St. Petersburg remained the Russian capital? How large would the butterflies be here? If the butterflies are small, how would having the capital in Leningrad effect WW2? Would Hitler focus even more on wiping the city out, as a symbolic measure?
1)The Russian capital was St. Petersburg under the monarchy, once they were otherthrown it was moved(the city was founded by Peter the Great, and thus a symbol of the Tsars, among other things).
2) How it would affect the interwar period is complex....I really have no idea. Actually, the white revolution may have more success if this was so, as St. Petersburg is easier for the West to reach.... Also, the Soviets may be quite determined in the inter-war period to annex the Baltics and vassalize Finland...... rather then mess around in Hungary, Mongolia and Persia.... and so we may see them succeding in Finland unlike OTL, and having a shot at the Baltics....