Common rail is a modular, Electronically Controlled Diesel Fuel Injection System.  The particular design of common rail, with its flexible division of injection into several injections (pre, main and post injections), allows the engine and the injection system to be matched to each other in the best possible way.

Common Rail Fuel Injection System is characterized by its High Pressure Accumulator (or Rail) from which it derives its name.  A high pressure pump develops the required pressure for injection and delivers it to the High Pressure Accumulator (Rail). The Injectors which are controlled electronically by the Electronics Control Unit draw the high pressure fuel from the Rail as and when commanded by the Electronic Control Unit.