You've probably seen this already:
"(current situation)
There is a rail service between Modlin Airport and E-65 railway line connecting Warsaw and Gdańsk which, as provided by the European Agreement on Main International Railway Lines (AGC), is a part of the Paneuropean Transport Corridor No VI connecting the Polish sea border via Działdowo, Modlin, Warsaw with the southern border of Poland.
(future)
The Polish Railway Authority (PKP PLK S.A.) plans to modernize Modlin railway station and Warsaw East (Warszawa Wschodnia) Railway Station - Modlin railway line. There is a siding from Modlin railway station to the airport which, after modernization (widening, improvement of track parameters, connection of electric traction, construction of the station building and related infrastructure) will be used for passenger transport and cargo loading/ unloading. Thus, the airport will acquire a railway connection with Warsaw and, ultimately, Warsaw F. Chopin Airport. "
From
Mazowiecki Port Lotniczy Warszawa-Modlin (but dated 2007!)
Edit: found this on TripAdvisor from February this year (which you may also have seen):
"Transport from Modlin airport in Nowy Dwór Mazowiecki will look more or less like this: there will be a shuttle bus between the airport and the Modlin train stop. From there, currently the train takes 45 minutes to get to Warszawa Gdańska station which is an interchange with the metro. Pole Mokotowskie is 5 stations away. However, there are supposed to be trains which won't stop at so many stations as the current trains, so that it will be no more than a 30 minutes train ride. There are also plans to open a new SKM line, from Chopin Airport to Modlin Airport, going through Central/Downtown stations. No details available as yet."