The railgun was revealed in an update from Game Informer that showed the arsenal of UNSC weapons that will be availble in the gun. The railgun is described as:
The ARC-920 Railgun is a compact-channel linear accelerator that fires a high-explosive round at incredible speed, delivering both kinetic and explosive force to both hard and soft targets alike.
Railguns are not new to the Halo universe but are typically reserved for capital spaceships and orbital defense platforms (MAC canons). Warthogs have been equipped with the railgun as well (Gauss Gun). However, this is the first time in the series that the powerful weapon has been included as something that Master Chief can carry around.
Halo 4 is scheduled to release on November 6 for the Xbox 360.
Via: Game Informer