There are several types of rail or blade type humbuckers. They are usually a side-by-side humbucking pickup with two bobbins that fit into a traditional single coil pickguard slot the size of a Stratocaster or Telecaster pickup. Joe Barden makes a beautiful hand made rail style pickup using a magnetic stainless steel polished to a mirror finish for Telecasters and Stratocasters along with other custom pickups. Joe had worked with Danny Gatton on his Fender Signature Model Guitar.
There are several other rail type pickups on the market that use a plated ferrous blade to conduct the magnetic field to the strings.