Thank you, that makes sense.I imagine Saltey was to the midlands what Newton Heath was to the north west. Secondmen from all over the country came to get booked for driving. This then created a web of massive route knowledge across most of the country.
RfD didn't, as far as I know (and I probably would have), put any pressure on Saltley LDC such as I mentioned for a TLF subsector above. Saltley was the best location for a lot of the traffic (Longbridge/Washwood/Lawley St) so no basis to threaten closure to get the rules bent.
It probably had fewer secondmen per drivers proportion than many depots, so, as you suggest, promotion was probably quicker.
RfD seems to have a bad reputation, with enthusiasts at least, but my own experience was that it was a good place to work and generally well-run - the best time of my railway 'career(?)'.