Bigger railguns FD ruled out a long time ago. They stated that they were testing it, but they found no way to get it balanced. They were unable to find any middle-ground between "feels too weak for a railgun of that size" and "does too much damage and instantly kills some ships".
And while the last may sound like fun for players, remember that it only sounds like fun while you're the one using it. You might feel different when you get insta-killed by somebody, let alone a NPC with a ship full of railguns.
And if the problem was already too big for them to handle at old times, now imagine how things would be when the first NPC with fully engineered C4 railguns comes around. (Without ever caring for heat, NPCs are disposable and don't mind boiling their internals. )