Caterham will only supply the new type of rail. It will fit, although if like me, EU2 R300 engine, I have to source another bolt for the front of the rail, as the original won't be long enough as the fittings are different. They also sold me another two sensors, as they told me the threads were different in the new rail. I have an MBE ECU so think that maybe a different thread, not sure on that point so went with CC recommendation, it all fits and the leads were long enough.
You will also need a new gasket. I did try and get my old one welded up before I bought a new one, but mine had a crack around the mounting bracket and after two attempts it still leaked, so went for a new one.
If going for a new one, make sure the gasket end of the rail is flat, then when you mount it, do up the gasket end first, then pack out the front bolt with some washers if needed, so when you tighten it, it doesn't force the rear gasket surface to twist, if you see what I mean