The railgun is a heavily modified RoTT weapon. In fact, from memory, it is mainly made from 3 of the RoTT weapons chopped about and re-joined to each other in a sort of Frakenstein-esque way.

I did that myself, then did a bit of re-colouring and adding flashes and so on. I think the flash was based on the Doom shotgun one.
As people may have guessed, some of the stuff I included in this WAD has actually come from my Aspects mod. So there wasn't too much work to be done creating resources for it. It was more a case of selecting which of the resources I had already made I was going to use.