Talking about the Duncan P-Rails (SHPR-1) here :
and, even cooler :
Basically we're talking about a real P-90 in a humbucker frame, with an added half of a hot-rails PU (but A5 instead of ceramic).
This gives you 4 completely different sounds, governed by two push-pull pots :
-- P-90, completely authentic
-- serial HB, fat, tight, punchy HB tone
-- parallel HB, same but not so loud and more transparent
-- rails-SC, decent alone but great in the middle position, not unlike a Tele middle position
And they clean up great from the guitar's volume pot.
About the most quiet PUs I've heard yet, all positions are noise cancelling with the exception of the P-90 and rails B or N alone.
Certainly not for everyone, blues- and classic rock players will be more likely to enjoy them than metal guys, but you never know.
Ymmv,
susiwong