Not speaking with any direct knowledge, but I know that automated diagramming software looks for the most productive solution overall, rather than necessarily at individual depot level. You may find that the optimal ‘all GWR’ solution for Dec 19 (with full timetable across the board) counterintuitively required a productivity loss at Plymouth to make bigger gains elsewhere. I imagine maintaining route knowledge on all the various routes to London is challenging and would possibly complicate matters for Bristol and Exeter depots?
I can't speak at all knowledgeably of automated diagramming software, but I can about the routing optimisation used by couriers, which is ultimately the same problem (combinatorial optimisation, and various relatives of the Travelling Salesman Problem).
The result is only as good as the rules that govern it - which is to say, not very. There are massive inefficiencies inherent in the modelling used by couriers, which is a much bigger and more lucrative market. I would be very surprised if that wasn't also true of automated diagramming.
The courier companies are of course continually refining their algos, but my experience is that the ones with the best customer satisfaction are those that give a certain amount of flexibility to their drivers rather than forcing them to slavishly follow an automated plan. In particular, the algos generally won't find permutations that
almost fulfil the requirements but not quite, so they'll drop an efficient potential solution because it's 1% off some target or other – whereas a human will intuitively think "oh, that'll be all right" and choose that.