Railgun

Railguns, also known as rail guns or rail cannons, are a class of electromagnetic cannon, used when infantry fielded coilguns (though simply called Gauss rifles and Gauss pistols in game, both coilguns and railguns are Gauss guns) are not powerful enough. They fire much larger rounds compared to coilguns, and are anti-materiel as opposed to anti-personnel.

X277 "Viper" magnetic rail cannon
Developed for the United States Armed Forces by West Tek, this railgun uses energy cells to propel a depleted uranium round across a series of magnets, discharging it at an extremely high velocity. Deemed too costly to produced en masse, the project was abandoned in 2044. On loan from the International Ordnance Museum, the only prototype was slated to be displayed from August 14 – December 31, 2077, in the Museum of Technology. Its display case is empty by August 17, 2277. 

15mm ARTEMIS rail gun


Artemis was an unfinished weapons project by Poseidon Energy and the Enclave, designed as a late-game weapon that could be crafted by characters with a high Science skill, using schematics from Mesa Verde. 

Behind the scenes
While not present in Fallout: New Vegas, game files suggest light rail cannons and heavy rail cannons may have at one point been intended to be in the game, with a unused script for the Miss Fortune perk mentioning both weapons.