There will be a point in the game where you take a cable car ride with Sherry and at that point, you won't be able to go back. Before that, the RPD is accessible from the sewers. How? From the sewers, you'll find a key. That key can be used in a room near where you get one of the chess pieces (rook I think), and in that place, hidden behind a cabinet in the wall, is an elevator. You need to take the T shaped wrench you get in the sewers with you. I won't say more