I'm thinking of my next Europe trip and leaning toward the above itinerary. Is it possible to go to Krakow then Warsaw and jump a train or cheap flight to St. Petersburg? Also does anyone know if this is a better trip to do during the spring or fall? Any info is helpful. Thanks a bunch!
There are no discount airlines that operate between St Petersburg and Poland. Your best connections would be via Finair-Helsinki, KLM-Amsterdam, or Lufthansa-Frankfurt. In terms of the weather, Poland is fine in the spring, but St Petersburg is much farther north, and has a longer winter. I went to St Petersburg in early March of this year, and it was snowing like crazy the entire time I was there. I went specifically for the Hermitage Museum and choose March so I could enjoy the museum without any crowds; it was a great experience. But if you are interested in visiting some of the palaces outside the city you are better off going during the warmer months. Don't forget you need a visa and hotel invitation to enter Russia
Krakow to Warsaw by train is easy, but Warsaw to St. Petersburg is a LONG ride, probably more than 24 hours by rail. I traveled in the opposite direction a year and a half ago, with a few days in Vilnius between St. Petersburg and Warsaw. If you really want to ride the rails from Warsaw to St. Petersburg, consider breaking up the trip with a stop in one of the delightful Baltic capitals... but its possible that only Vilnius has a direct rail link.
April weather in St. Petersburg can also be surprisingly cold. I think spring weather really doesn't start until late May.
Finally, I would recommend using a good travel agency for visiting Russia. Although it is a fascinating country, there is more red tape traveling there than anywhere else in Europe. I gave up trying to understand it all, and just booked through an agency.