The New York Pneumatic Railroad (also known as NYPR and New York Pneumatic Railway)  is the name given to the abandoned subway line running beneath New York City, found by the Ghostbusters after uncovering one of its air-shafts on First Avenue.
Details about the Pneumatic Transit system are scarce, save that it was constructed c.1870 and then abandoned for unknown purposes. Only one station on the system is seen, called Van Horne, which lies beneath First Avenue, and can be accessed via an air shaft from the surface or via a service tunnel leading from the modern subway system. No other stations are mentioned or seen.
Van Horne and the entire Pneumatic Transit system were constructed in the 1870s by inventor Alfred Ely Beach, and was referred to as the New York Pneumatic Rail Road (NYPRR). The air-shaft opened by the Ghostbusters bore these initials on a man-hole cover covering up the shaft. Egon Spengler explains to Peter Venkman it was an experimental subway system that was to use fan-forced air-trains, trains blown along the tunnels by gigantic fans, and that the line was an experiment in providing a new, novel means of mass transit.
Ray Stantz noted to himself the NYPRR was a shortline railroad in use before the subways. Shortlines are small to mid-sized and are used to link up two industries, interchange traffic with a larger railroad, and tourist transportation.
The River of Slime
For unexplained reasons, the system was not a success and was subsequently abandoned, its tunnels being sealed up and forgotten by the outside world.
Sometime after the First World War, the Cult of Gozer created Psychomagnotheric Slime, which somehow flooded the abandoned subway tunnels and began flowing steadily through them, leading towards the Manhattan Museum of Art to provide power to Gozer but it was later used by the spirit of Vigo, whose spirit was residing in a painting there.
Exactly how the slime first entered the tunnels, or whether the subway once had a station close to the museum, is never explained.
Behind the Scenes
The scenes were filmed on Stage 16 at Burbank Studios.  The scene with Ray being lowered into the Van Horne station was a combination of a partial set and matte paintings. Bo Welch built a small section (the curved background wall, steps leading down into the station, and a partial platform). The set was then combined with matte paintings by Yusei Uesugi.  For the scene where Winston, Egon, and Ray plunge into the river of slime, a partial set of the Van Horne station was used. Ernie Hudson would fall to be followed by Harold Ramis and Dan Aykroyd. Off screen, the actors would land on airbags. During optical compositing, Industrial Light and Magic merged the live action with their miniature river. The part when the river sweeps them away required more finesse roto work and alignment. Pat Myers had to pinblock a bluescreen element of Ernie Hudson against the background and trace his movement in the river with the subtle movement of the slime taken into account. Sean Turner then animated the rippling edge around Hudson. Soft edge work and composure shifting was done to make it look like the river was enveloping the actors. Smoke was put into the plate, some articulate work was done by animation, and matte paintings of the archway were then added. 
- In Ghostbusters: The Video Game (Realistic Versions), some work was done on a level featuring the Pneumatic Railroad and the River of Slime but it was not used for the game.
- On page 18 of Ghostbusters 101 #6, the fifth image is from when Ray was first lowered down from First Avenue and found himself above the River of Slime. Patty Tolan appeared in place of Ray.
- IDW Comics
- Ghostbusters 101
- Insight Editions
- Egon Spengler (1989). NOW Comics- "The Real Ghostbusters starring in Ghostbusters II Part Two" (1989) (Comic p.20). Egon says: "The New York Pneumatic Railway. It was an experimental subway system. Fan-forced air trains built around 1870."
- http://en.wikipedia.org/wiki/Alfred_Ely_Beach Alfred Ely Beach
- Naha, Ed (June 4, 1989). "Ghostbusters II" Mass Market Paperback, p. 48. Dell. ISBN 0440204607.
- February 27, 1989 Draft, Pages 27-28
- Van Hise, James (October 1989). "The Real Ghostbusters Starring in Ghostbusters II" No. 1, p. 22, panel 4. NOW Comics, Chicago IL USA.
- Ghostbusters II: Junior Novel, Pages 24-25
- Shortline Wikipedia Article
- Ghostbusters: The Video Game
- Spook Central California
- Eisenberg, Adam (November 1989). Ghostbusters Revisited, Cinefex magazine #40, page 9. Cinefex, USA.
- Eisenberg, Adam (November 1989). Ghostbusters Revisited, Cinefex magazine #40, page 26. Cinefex, USA.
- Ghostbusters News 3/17/2010
- Slime Tunnel -deviantArt User:Kuren 4/19/10
- Narrator (2016). Insight Editions- "Tobin's Spirit Guide" (2016) (Book p.64). Paragraph reads: "A veritable river of the stuff had built up over the years in one of New York's many abandoned subway tunnels, a byproduct of the high amount of spectral energy ever present in the metropolitan area. (It is our supposition that only the positivity brought in through the tourist trade and professional sporting victories has kept Manhattan from being consumed and destroyed in a flood of burning ectoplasm)."