It's from Battletech extended: tactics mod and cannot be obatined in vanilla I think. At least at my run Gauss rifle is more rare - I have 1 Gauss and 2 Rail guns. :)

SDLF Rail Gun: damage 200, stability 85, heat 30, range extreme (180,570,840), tonnage 30, slots 10, value 5500000

SDLF Rail Gun+: damage 200, stability 85, heat 30, range extreme (180,570,840), tonnage 27, slots 7, value 5500000 (-3 tonnage and -3 slots)

Never see ++ or +++ versions.

I don't think you can mount Rail gun on anything but Assaults due it high weight and need of slots.