What exactly does the Network Rail (?) archive Working Time Directory cover? How far does it go back, and how do you search for a specific location?
Essentially you've got to go and look for yourself. Working Timetable also.
If you're familiar with working timetables then you'll have some idea of what area each of them covers (CJ for the Midland Main Line, for example) but if you're not you'll have to work it out for yourself I think!
Cx London Midland Region
Yx Eastern region
Lx East anglia
Px Western region
Gx Scottish region
Wx Southern region
to get you started.
But the archive covers - from what I have seen - all working timetables for the whole of the network for a number of years, which in my case fill in the gaps between my printed working timetable collection and the more recent PDF download collection.
EDIT The timetables are scanned, from what I can tell, from actual timetable books. From some time such as 2008 onwards they were published as electronic PDFs, but the period of the archive covers the period when they were still printed. My most recent printed book is from May 2007, so at some point after this date the printing stopped.
Also see
https://www.networkrail.co.uk/running-the-railway/the-timetable/working-timetable/ for Working Timetable Maps which show the area each of today's working timetables covers, with the caveat that the working timetables from the 1990s didn't necessarily each cover the same area as today's timetables.
Western Sunset helpfully posted something which got left in the original thread:
For those who wonder at the nomenclature of the WTTs:
C = Crewe
Y = York
L = Liverpool Street
P = Paddington
G = Glasgow
W = Waterloo
This two-letter naming convention came in in 1975; earlier than that each region just called its timetables "Section A", "Section B" and so on. From about 1990 the area covered by each timetable sometimes changed, so for example the South Manchester area which used to be in timetable CM was now to be found in timetable CE. More recently again, from around the time the timetables stopped being printed and were issued as PDFs, say 2006, the naming has gone back closer to the 1970s naming conventions.
Also, the link to the Network Rail Working Timetable Collection is
https://history.networkrail.co.uk/uncategorized/SO_9305941a-b249-44ea-bc39-d988585d6e81/, which contains scans of working timetables from 1994 to the present day.