The Pipeline is a hairline fracture somewhere on the border of Purgatory and it leads back to the physical plane.
The Bosses made a deal with the Demons to gain access to the Pipeline in exchange for giving him souls off the record. The Bosses started a coyote operation and smuggled souls back to the real world. Once through the Pipeline, the souls would be transported to an underground subway track in New York City. The forces of Heaven became aware of flagrant, recurring border violations and sent J. Edgar Hoover, Jelly Bryce, and Elliot Ness to Purgatory and investigate. It appears the Yakuza Ghosts might have taken over the Pipeline after the Bosses were trapped by the Ghostbusters but it is unknown if it was ever shut down for good.