The Subway Ghost was one of many ghosts that escaped from the Containment Unit.
It's unknown when the Ghostbusters encountered this ghost, but at some point after their first bust, they caught it and placed it in their storage facility. When Walter Peck barged into their Firehouse with a warrant, he forcibly shut down the facility's protection grid, causing the Containment Unit to explode, releasing all the ghosts the Ghostbusters had caught. Upon being freed, the Subway Ghost retreated to a subway. It emerged at street level, flailing his arms and mouth tentacles and wailing and howling. It frightened away surrounding citizens in the process.
Legion Mini Series
The Subway Ghost was one of many who came under the control of Michael Draverhaven. He sent the ghost to terrorize passengers in a subway train as part of an information gathering session. The Ghostbusters rappelled down onto the roof of the train and chased after the ghost inside. Suddenly the ghost stopped and its ectoplasmic cortex shorted out. The Ghostbusters easily trapped it. Michael observed some of the Ghostbusters' tactics and weaknesses from this bust.
During the Tiamat incident, the Subway Ghost was released when the Containment Unit was vented. Three months later, it haunted the Saks Fifth Avenue flagship building. The Ghostbusters took three hours to capture it. Upon leaving the building, Winston Zeddemore saw his former wife Tiyah Clarke, whom Tiamat stole a portion of her soul and erased a chunk of her life experience from the world, at a Pequod's nearby. Peter Venkman elected to let the ghost go so that Winston could save Tiyah and rekindle their relationship.
Originally, the Subway Ghost was designed by Brent Boates as a three headed creature. It was deemed too expensive and time consuming to create the rubber-suit for just a brief amount of time on-screen.  Instead, Steve Johnson sculpted a "small non-humanoid flying creature" as the replacement.  The finished model was cast in a hot melt vinyl material, which provided flexibility, with an armature constructed from brass and aluminum. It was was painted with pearlescent powders then filmed in a large water tank in order to have it float properly. It was filmed in in reverse. This was achieved by attaching monofilament to the tail and pulling the puppet backwards. By reversing the film, they made it appear as if the ghost was flying towards the camera.  The filmed material then went through additional diffusion and optical alteration. It was then composited into the corresponding live-action scene shot in New York. 
- The scene with the Subway Ghost was shot in October 1983 before principal photography. 
- Cruster, a ghost in Ghostbusters: The Video Game, bears a resemblance of sorts to the Subway Ghost.
- 88MPH Studios
- IDW Comics
- ↑ Shay, Don (November 1985). Making Ghostbusters, p. 144. New York Zoetrope, New York NY USA, ISBN 0918432685. Paragraph reads: "The original subway ghost - as rendered by Brent Boates - would have been a major rubber-suit assignment for the 'ghost shop,' and was ultimately deemed just too expensive and time-consuming for its minimal on-screen time.'"
- ↑ Shay, Don (November 1985). Making Ghostbusters, p. 144. New York Zoetrope, New York NY USA, ISBN 0918432685. Paragraph reads: "Instead, a small non-humanoid flying creature - being sculpted by Steve Johnson - was devised as a replacement.'"
- ↑ Craig Caton-Largent deviantArt "Ghostbusters Subway Ghost" 7/7/13 Paragraph reads: "The Subway Ghost from Ghostbusters inside the cloud tank. This was made from hot melt vinyl with an armature constructed from brass and aluminum and was painted with pearlescent powders. The shot was filmed in a "cloud tank" in reverse by attaching monofilament to the tail and pulling the puppet backwards. After reversing the film, it appeared to be swimming towards the camera."
- ↑ Shay, Don (November 1985). Making Ghostbusters, p. 144. New York Zoetrope, New York NY USA, ISBN 0918432685. Paragraph reads: "To achieve a suitable floating effect, Jon Schreiber maneuvers the finished creation - cast in a flexible vinyl material - through a large water tank. The resulting image would then undergo additional diffusion and optical alteration before being composited into the live-action scene shot on location in New York.'"
- ↑ What Happened Next! (2014). IDW Comics- "Ghostbusters Volume 2 Issue #20" (2014) (Comic p.24). Sentence reads: "And after Winston saved her from a nasty little Class 5, she invited him out for coffee."
- ↑ Ivan Reitman (2005). Ghostbusters- Commentary (2005) (DVD ts. 1:09:26-1:09:30). Columbia TriStar Home Video. Ivan Reitman says: "This is all part of that very early shooting we did back in October as I remember. "