Bobby's bus is an old bus that was parked at Singer Salvage Yard in Apocalypse World.
For an unknown period of time, the bus was parked at the Singer Salvage yard of Apocalypse World. Due to the length of time it was parked, it eventually became overgrown with vines.
While trying to figure out a way to transport dozens of people to the rift to the Main Universe to escape Apocalypse World, Dean spotted the bus and asked Bobby about it. Over the next few hours, Dean repaired the bus with the help of Lucifer and managed to get it running again with just under two hours left until the rift closed.
Once everybody was set to go, they boarded the bus which was driven by Lucifer with Bobby hanging out the front door and Mary sitting in the rear door to act as protection in case of an angel attack. Guided by a jeep driven by Sam, Dean, Gabriel and Jack, the bus carried the evacuees to the rift, arriving just as it was about to close. With the rift held open by Rowena MacLeod, the evacuees quickly abandoned the bus to race through the rift to the Main Universe.
The bus was ultimately left abandoned alongside the jeep in northeast Kentucky by the site where the rift once stood.