We are looking WAY into the future, but were curious about an excursion on Oceania Cruise Lines (or similar) between St. Petersburg and Moscow. It appears that it is a about a 4-hour train ride, with an all- day tour of Moscow ending with dinner before you head back to the ship on the train. Is this way too much to do in one day?

It's a four hour train journey if you take one of the express trains (Sapsan or Nevsky). Moscow deserves much more than a day trip, it is a huge city with an extraordinary amount of sights to see. It would be too much for me. I spent a year in Moscow and felt it wasn't enough, never mind a day trip. But if that is all you can do on your trip it is better than not going at all.

Eight hours on the train (you have to return) in order to spend a few hours rushing round a city that easily deserves a few days? Not for me, but YMMV. There's also the question of how much time you will have in St. Petersburg, another city that deserves several days.

Yes, it is an insane itinerary.

Several years ago we went on a Princess cruise which spent 2 nights in St Petersburg. Princess offered a day trip to Moscow.
It was by airplane (1hr30m each way). There was no high speed train at that time. We chose to spend the 2 days in St Petersburg and surroundings. We had a great time in St Petersburg.
The closest we came to doing something like you are thinking about was a cruise which stopped at Alexandria, Egypt. We went on a 2 day (overnight) excursion to Cairo (100 mile bus trip ). We returned to the ship at Port Said (122 mile bus trip) We saw a lot of Cairo in 2 days.
One day with 8 hours on a train, no thanks.
Perhaps you should look for a cruise that offers a 2 day excursion to Moscow $$$ to check your bucket list or just enjoy St Petersburg.

Take a Russian River Cruise with several days each in Moscow and St. Petersburg.

I agree.