I am thinking of flying from Warsaw to Moscow. The problem with the cheap airline air Baltic is that it arrives at
2:00 in the deep night.
Is there still subway to the downtown?
At that hour, I presume you are flying Air Baltic BT 416 from Riga to Vukonovo. You can see the routes on this route map , enter Moscow on the left and pick SVO or VKO to see the flights. The only bus is 611 which does not run per this 611 timetable this late. The express bus is even more limited per this 611s timetable.
If your flight arrives in Sheremetievo (BT426), then you may use the H1 (it's russian N letter) night bus. It goes right through the city center, and at night there will be no traffic jams.
Judging by the timetable, the first bus leaves the airport at 03:00, which should give you the time to pass the customs, then every half hour.