Belarus Station, the Moscow rail terminal for trains from Belarus (Minsk and Brest), Kaliningrad, Smolensk, Vilnius, Warsaw, and Berlin, is located a little over 3km north from the Assambleya Nikitskaya Hotel.
By taxi
If you are willing to try negotiating a cab on arrival at the station, the cost of the journey should not be more than $20. However, many may be more comfortable booking a transfer with a driver to meet your train in advance, which you can do here. Either way, the drive will take around 20 minutes in moderate traffic.
By metro
From Belorusskaya Metro Station (connected to the mainline station concourse), travel three stops south on the dark-green line to Teatralnaya, which is roughly 400m from the Assambleya Nikitskaya Hotel.