As someone who works at PRDC (Willesden), I can offer the following insights:
Royal Mail are increasingly moving away from conveying mail by anything other than trucks. This is nothing to do with engineering works; it's purely down to cost (track access charges too high?). The rail hub had 7 platforms when built, but four of these (and the ‘roads’ between them are covered-over and the space is now utilised for mail processing. Indeed, a building-within-a-building was constructed, with staff shielded from the wind and rain that would otherwise give intolerable working conditions in winter.
When the WCML was flooded near Carlisle a couple of years back, the 325s ran via Camden Road and the ECML for more than a few weeks. Obviously, the third-rail shoes weren’t needed, as it’s all ‘under the wires’ going that way. But it implies drivers do have full route knowledge!
There is a still a sign near the slip road entrance that mentions Railnet. I’ll try to take a photo and upload it, if anyone is interested.
You can see a lot of ‘runs as required’ paths, in addition to the ones actually used, by going on realtimetrains and inputting XRN as the station / location !