My understanding was that the second (straight air) system was installed as it was easier to control the train with in the course of normal operation, pretty much why it was chosen for use on the DRR. The automatic air is the "failsafe" system. Handling a steam train on a mountain road like the D&S (or the CATS) with automatic air takes a lot of experience and a lot of skill. With the advent on dynamic braking on Class One roads it's something that is almost a lost art.