I have taken the "fast" train from Moscow to Sankt Peterburg several times. As I recall it leaves Moscow at somewhere around 18:00 and arrives SP somewhere around 22:30. I believe there is a similiar "fast" train that leaves SP @ 18:00 and arrives Moscow 22:30, although I have not taken this one.
This train cuises at speeds of 200 km/hr on the open road.
When one considers the city center to city center locations of the train stations it is more convenient than flying in most cases. Given the added expense of taxi's to and from airports and added time for security and so on, I believe that one saves not much time by flying this one hour route. Also, it is a nice experience to take the train.
As mentioned I am not certain that the "fast" train goes from SP-Moscow, but know it does from Moscow-SP. As I recall some Russian friends menitoned to me that the "fast" leaves SP and arrive Moscow at or near the same time schedule and the mentioned "fast" train from Moscow to SP.
Perhaps someone can confirm this for you. Also, as mentioned, there are several other trains schedules between SP-MOS.