The fuel rail supplies the injectors with fuel. It is important that the fuel has the necessary pressure at the start of the injection and that there are minimal pressure fluctuations between individual injections. A fuel rail is high pressure tubing which takes fuel to the injectors in an internal combustion engine. The fuel passes from the fuel rail into the injectors in metal fuel pipes. The fuel rail is the long, round metal bar that runs along the top of the injectors and is held onto the manifold with a few bolts.