You can do Paris-Berlin-Moscow-Beijing by train via the trans-Siberian, or via Ulan-Bataar with the trans-Mongolian.

Quite an iconic route that became much simpler administratively to travel in 2000s but perhaps again trickier now because of the situation with Russia.

You can't, unfortunately. No trains run between Poland and Belarus.

Oh that's a shame... I believe the whole journey was not an issue during the "warm period" of the 2000s.