A rail pickup uses a single rail in place of a row of six pole pieces in each coil of the pickup. rail pickups are available as single-coil or humbucker designes, with rail humbuckers available as full-size humbucking pickups, in narrow side-by-side humbuckers that fit in the space of a single-coil pickup, and in stacked humbucker pickups.
One advantage to a rail pickup is that the strings never leave the full magnetic field of the pole piece, even when a string is bent. With “conventional” pickups, a bent string can end up between two pole pieces and may drop in volume or change tone due to the change in magnetism. Another advantage is that string spacing is not an issue; no matter the string spacing at the bridge or nut, each string will be properly aligned with the rail.
A disadvantage of a rail is that it is impossible to adjust the response of the pickup on a per-string basis. With a “regular” pickup, each string has its own adjustable pole piece(s) that can be used to balance their volume. A rail can only be adjusted “globally”; bass side up or down and treble side up or down.
