Their is a railgun on the wall at mick and ralphs
Even with console commands it's only for show, doesn't work.
On topic:
Im guessing here, but I think they add all types of weapons into a DLC so certain players don't feel left out. Which means we will probably be getting a new Energy Weapon in HH too (Unarmed, Melee, and Guns go without saying. Unsure about explosives.). I'm all out of ideas of Energy Weapons they could conjure up, I wonder what we might get?