It is thought that Imperial forces of Wh40k do not make use of Railgun, since their purging power is not exactly great (after all, you can't exactly have a railgun flamethrower), but if I recall correctly, one of multiple explanations of Nova cannon is that it's an huge railgun. Some places mention that heavy bolters are also coilguns (which makes them have a kicker charge, speed up the projectile by coil and then have a rocket on top of it, because three ways to make a bullet faster is better than one or two... if they only painted them red) but that's very obscure knowledge.
Necron "Gauss" weapons are not railguns. They are reverse railguns, since they pull the thing they're shooting at towards themselves, effectively "flaying" the enemy and ripping him apart one layer at a time, which is also why they're quite useful against vehicles. It should be noted that Necrons also utilize "Tesla" weaponry which basically shoots electricity because that obviously makes sense. One could argue that they are named like that because Mechanicus have a preserved skull of Nikola Tesla that they use to shoot lighting out of - which implies that they might have skull of Carl Friedrick Gauss and use it as a gravity gun.
Of course, Tau make heavy use of railguns but their railguns are so simple and regular that there is not much to write about, apart maybe from Cluster Shell system from Heavy Rail Cannon Arrays which allow them to execute multiple human civilians at once.
Since I have noticed the new name only just now, what about we discuss some other weaponry, like
chainswords with flamethrowers underneath? This is the most holy combination of two weapons that are most useful for righteous purging, why it is not used more? Catholic Church would certainly benefit from priests armed with Eviscerator chainswords.