Reinforced Rails are a booster to projectile weapons. They increase the shield bypass and add speed to the projectile weapon's slugs as well as decreasing the minimal range arc. They are relatively rare to be seen in combat given how ineffective projectiles weapons are high level combat. They are exclusive to cruisers only.
They synchronise well weapons such as Storm drivers or Gemini drivers and to some extent, Gladius drivers, since reinforced rails is designed to augment such weapons at medium range combat. But most projectile weapons lack firepower and only try to make up for this with their unique damage type and range.
Reinforced rails is a unique hybrid of Iridium Magnets, minimal range reduction, and phased projectiles. It is, in effect, a combination of a range deduction with phased magnets.