I mean, 8d8 is some pretty nice damage, sure. But what sets it apart, is that I THINK (Do not quote me on this, please) that the Railgun completely ignores armor when it shoots.
In DoomRL, armor is straight-up DR (damage resistance), such that 1 armor means you take one less damage per damage source. For a weapon like the chaingun, whose normal damage is 1d6, it can be a big problem. For a weapon like the Railgun, whose normal damage is 8d8, armor is barely noticed at all.
According to the wiki, it does plasma damage, which halves armor.
This is true (to be extremely technical, only half of the armor (rounded up) is applied when the damage source comes from a plasma type), AND it applies to enemies as well. Cacodemons and Arachnotrons, at the very least, deal plasma damage, which is part of the reason Arach caves are such an annoyance.
Along the same vein, shrapnel damage (from shotguns) doubles the armor applied, which is why double shotguns aren't as awesome against Revs as you'd want them to be.
But yeah, Railgun is one of my favorites, even if I've only seen it once or twice in a real game. What truly sets the weapon apart is the railgun-effect: hitting several enemies simultaneously, with precise aim, is unparalleled in usefulness. Hell, I usually get rid of my rockets and just hoard plasma 'cause of the wall-destroying potential. Just a great weapon in general.
