It depends on the train. Düsseldorf Flughafen, on the east side of the grounds, is the long-distance station — “more than 350 trains… from S-Bahns… to ICEs” a day — but it is not inside the terminal: you ride the SkyTrain about seven minutes to get there. Düsseldorf Airport Terminal is a separate, S-Bahn-only stop served exclusively by the S11 line, sitting in the basement of the terminal building itself, no monorail required. If your ticket says S11, walk down; for everything else, budget the SkyTrain ride.
