they function under high gain much better.

cool things about them, you can bend and never lose the note, because the rail picks up the vibrations everywhere.

dual ones, it looks like the humbuckers with two single coils together, with each having one rail on it. when in humbucking mode, they function like a normal humbucker. when switched to single coil, they function like a normal single coil. (no hum reduction) as for quad, they look the same, but have two rails on each half of the pickup. so even if they're switched to single coil mode, they still will have a humbucking effect.

sometimes thats desireable (i.e. metal) sometimes not. (blues, etc.)