Actually CAP 509 is just a book (Civil Aviation Publication #509) - but it became a generic term to name the course of training that the book was all about.
Well - it's dead.
It used to be what is now known as "integrated training" - where wannabes (with zero flight experience) would wander off to residential pilot school and go from zero to airline job in 13 months of intensive training.
The course was geared specifically to put pilots directly into the right hand seat of an airliner - straight from school - and it worked.
Some years ago, the American think tank set out to survey all the training regimes in the World to see which one was best. The UK CAP 509 course was the winner (even over their own FAA Part 141 course) - so a trial was adopted to see if it would work for the FAA too - hence the birth of Western Michigan University Aviation Department.
As they say in the press, "The trial continues"