No transfer from St. Petersburg to the Bulgarian city of Varna on the direct train 051Ya "St. Petersburg - Varna". The train runs from January to September once a week, on Mondays. Departure from Vitebsk railway station at 19:13. The journey takes 2 days and 16 hours. The train follows the territory of Belarus, Ukraine and Romania. For transit through Romania, a transit visa is not required. In Varna, the train arrives at 10:55 local time.

Also you can go with a transfer, having previously reached Moscow, and from there you went to Bulgaria. From Moscow, trains depart from the Kiev railway station.

You can get to St. Petersburg from Bulgaria via Moscow. And the train schedule can be found on the Russian Railways website