Moscow and St Petersburg are ~700 km apart. It takes 4 hours by high-speed train, 8 hours (a night basically) by low-speed train you can sleep in, or about an hour by plane. And none of these are optimal. You have to spend those 4 hours awake if you're taking the high-speed train, you might get annoying neighbors if you take the low-speed one (and some people also don't fit into those berths), and all 3 airports in Moscow are in the middle of nowhere, so it essentially takes the same 4 hours if you fly.

And I'm not taking price into account, it varies a lot. I'm just ranting that there isn't a good option for this kind of distance. Not long enough to fly, but a bit too long for a train.

Oh and you're only left with the slow train if you want to spend a day, including the evening, in Moscow, and return at night.