Any idea why the GN sent coal this way rather than along their own line to London?
The line had an interesting history; originally it was an ambitious G.N. scheme to get to West London but, hitting financial problems, they were bailed out by the LNWR - who then exploited the partnership to their own advantage.The LNWR got a lot more out of the deal - e.g. running powers to York - than the G.N. did, and of course the LNWR were only interested in getting the line as far South as Welham Junction, where it connected into their network.
Coal merchants in London were based very locally (there were some big factors, but they still had local outlets (there were no 'Coal Concentration Depots' then), usually at stations of course, and these locations were on lines belonging to the different railway companies then in existence.The line gave the opportunity for the G.N. to serve coal merchants in areas served by LNWR lines - broadly in West (-ish) London as well as G.N. ones. It was all very complicated, some companies had goods or coal depots that they owned and ran, but which were not on their own line. Wagons were routinely sent to destinations on other lines, so the G.N. could have done this, but this meant, through the Railway Clearing House, that the originating railway paid a 'rival' for conveyance over the last leg. Much better to keep the revenue to yourself or shared on an equitable basis.
As far as the collieries went; there were three railways running down the Leen Valley (GN, GC & Mid) and although the lines ran close together and crossed each other each company tried to get a monopoly on a colliery to freeze the others out (which was why B.R. found it difficult to rationalise the lines, because collieries were connected to one or two of the lines in different permutations). The Joint Line gave, via the G.N. the L.N.W.R. access to a completely new source of traffic without having to build a fourth line.
Freight traffic has always been a complicated subject and the railway authors such as O.S. Nock were only interested in express passenger trains. There are less superficial books - I recently came across a biography of Mark Huish, an executive of the Grand Junction and then of the LNWR. It details how he negotiated with other companies the rights to and apportionment of freight traffic flows, all very interesting to me, but perhaps harder work to read. My interest stemmed from a childhood happily watching a procession of varied coal and merchandise trains clatter pass on mysterious workings from faraway exotic places (like Colwick!) while everyone else was watching 'boring' expresses with their routes displayed on carriage boards..
