Re: SD P90 in a H'bucker body?
Nothing is the same, exactly.
The P Rails Neck has the closest sounding P90 in humbucker shape that I've ever played. I have played many, searching for THAT tone. The P Rails P90 coil is better than the Phat Cat, for sure. Part of this might be because the coil is narrower than a Phat Cat (closer to a real P90 coil). Part of it might be because the coil ends up closer to the original P90 position in relation to the bridge, as opposed to being slightly farther toward the neck in the bridge pickup, and slightly farther from the neck in the neck position, like it is with Phat Cats and other similar pickups. Honestly, I don't know exactly why it sounds the closest. But I do know it's the closest I've ever heard. You don't have to use the rail in any form; the P90 coil itself is good enough to warrant installing the pickup.
Second closest IME is, oddly, the DiMarzio Bluesbucker.