Egon-1 is the primary vehicle used by the Peoplebusters. It is about the size of a tank and even dwarfs the Peoplebusters in size.
After the Ghostbusters initially fled from the Peoplebusters, they pursued the guys in a vastly oversized version of their own Ecto-1. However, the Ghostbusters flew right past it and the chase was renewed.
The actual driver's seats are a very tiny compartment up front, leaving the purpose of the vast bulk of the vehicle a mystery. It's likely used to transport their captured human prisoners similar to a "Paddy-wagon" since humans are shown to take up more space when captured compared to a ghost in a much smaller trap.