A Belarusfeed article from December 2018 notes:
Once the Belarus-Russia visa recognition agreement comes into force, foreigners won’t need two visas to stay in Belarus and Russia.
Belarus and Russia fully agreed on a draft intergovernmental agreement on the mutual recognition of visas on Friday, 30 November .
The Belarusian side expects the mutual recognition of visas to come into force by May .
However, the official website of the Republic of Belarus noted in April 2019:
BelTA reported earlier that Belarus and Russia were supposed to sign the mutual visa recognition agreement at a session of the Council of Ministers of the Union State of Belarus and Russia in Brest in December 2018. However, Belarus had failed to complete all the necessary intrastate procedures in time for the session.
which implies it is not yet in force for May 2019.
What is the current situation? Do I still need a Belarus transit visa when taking the train between Poland and Russia?