There is unit injection like the PD TDI's & then there is common rail, which is a different deal. The cam does not perform part of the injection process in a common rail system.
Bosch's common rail system works more or less like this:
There is a fuel delivery pump that supplies the fuel rail, or "accumulator" with fuel at full delivery pressure. The Bosch pump they show in their manual is a three plunger radial type pump. The fuel rail then holds the fuel at full delivery pressure to be used by the injectors. The rail holds enough fuel that when an injector uses some fuel for injection the pressure of the rail does not change. The rail also has a type of pressure relief valve that regulates maximum system pressure. If the pressure goes too high this opens & some of the fuel bleeds off into the fuel return system. The injectors are connected to the rail and are seeing full delivery pressure all the time. They are balanced so they stay closed even with that much pressure on them. The injectors have solenoid valves at the top & are all connected to the ECM. The ECM sends an electrical signal to a solenoid to tell it when to open & for how long. The magnet force of the solenoid is enough to change the balance in the injector & allow the needle to lift off the seat & then fuel injects into the cylinder. The neat thing is that this opening & closing is all controlled by electrical signals rather than fluid pulses from a pump like our TDI's have. This allows the system to have things like pre-injection, main injection & then in some applictions post-injection. Some of these Bosch systems are rated up to 29,000 psi of injection pressure! /images/graemlins/eek.gif
There are other companies like Siemens that make common rail systems & I'm not sure how different they are. If you're a tech weenie & want a good read try this:
It's $16 or so at Robert Bentley Publishing.