Rail Guns in the Galactic Union are space-based relatives of the ground based magnetic accelerator cannons, though they are larger with a slower cycle rate. They use powerful magnetic buffers (related to the ones used in PA and fusion toroidal reactors) to accelerate a package to speeds as much as .1 C. Despite the fact that they are easy to produce, they are not in use anymore.
They are mounted in turrets that sit along the hull of the ship and are extended in combat. They are often hundreds of meters long, allowing incredible muzzle velocities. Because they are fired in space, there is very little wear on the weapon system.
They can be extremely devastating, delivering multiple terawatts of impact energy. 'Shot' packages containing hundreds of small penetrators are devastating against ships because shields are relatively ineffective against small ballistic objects.
Rail guns ratings are expressed in kinetic impact energy through wattage. Impact energy start at a modest 10-megawatt, and can reach as high as 1,000 gigawatt for Dones found aboard dreadnoughts. They are the only weapons system that requires less power than is yielded, genreally 75%.
A rail gun's failure is usually a result of computer glitch since the magnetic accelerator is extremely sensitive and must be triggered in careful order, especially on the larger, faster versions. They are vulnerable to laser fire as their long barrels often extend outside of shield protection. Also, the packages they fire are expended, like missiles, and require a magazine and loading mechanism.
Rail guns tended to be found on battle cruiser and larger ships. They were extremely common on dreadnoughts where they would be mounted on clusters for use on planetary siege. Each package produced multiple megatons of energy on impact with little residual radiation.
As mentioned, they are not in use today. The Union ceased their use via a general agreement as they proposed a unique danger to shipping in star systems were battles had taken place. A package fired 500 years ago could still inflict damage on an unwary ship. Due to their velocity and small size, there would be very little warning of encountering an ancient battle. Examples can still be found in places, most often with non-Union member states.