I am just looking over these again. The idea of having sleeping carriages is most interesting, as noted above, and may become more important if/when comfort based routing is implemented as is eventually planned. These carriages do look good (although I have darkened the wood shade in the .blend for the NBR carriage as the existing wood was too light).
All other things being equal, it is probably a better thing to have sleeping carriages in the game than not; but one question that we have to consider is at what stage of completeness of the work of adding sleeping carriages is it worthwhile to add this work to the pakset? We have, at present, a carriage from the 1870s, some from the LMS (1920s) and some from BR (the 1950s-1970s). Assuming each type of carriage to last circa 30 years, then the period covered would be (to the nearest decade year) 1870-1900, 1920-1950, 1950-1980, 1970-2000. If we want minimal coverage of all eras, we are therefore missing 21st century sleeping carriages (the mk. 5 design for the Caledonian sleeper, introduced this year) and early 20th century sleeping carriages. If we want a somewhat more realistic coverage to take into account the fact that new designs would have emerged somewhat faster than the slowest replacement cycle, we would probably need to have designs from the 1880s/1890s and possibly 1910s and 1930s, and it would also be good to go back in time to the 1830s when sleeping carriages first started. We would not, I think, need coverage of more than one railway company's sleeping carriages for there to be a minimum to warrant integration, but it would of course be good to have this in time.
Thus, in order for it to be sensible for me to implement these, the absolute minimum further work would, I think, be two further sets of sleeping carriages: one from the early 20th century (at your discretion) and the latest mk. 5 sleeping carriages (and if you are doing the sleeping carriages, you might want to do the mk. 5 day carriages, too).
You might then want to fill out the sleeping car time line by filling in gaps in the late 19th century and the 1930s and then go back earlier in time to the Grand Junction Railway's bed carriage of the 1830s. I can supply research material for these earlier times if that would help.
Turning to some of your more specific questions: it is correct that there are currently no North British Railway vehicles of any sort in the pakset. This is because my policy in filling in details has been to try to have a coherent set of a few railway companies' vehicles rather than to have a smattering of content from a large number of different railway companies. So far, I have not embarked upon any work involving the North British Railway. It would certainly be good to have more North British Railway content as Scotland is very under-represented in railway terms at present. If you would like to take on the work of providing North British Railway carriages and locomotives, that would be splendid.
As to your question:
Does the following statement apply? "East Coast Joint Stock was varnished teak, whilst Midland & North British Joint Stock followed the Midland's coach livery of crimson lake." ((url=https://www.lner.info/co/NBR/livery.php)link(/url)) I'd say that they would try to stick in line with Midland on this one for the long trip to London, but input would be appreciated.
As I have not researched the North British Railway, I do not have any independent sources to confirm or contradict that, but I do know that, in general, carraiges that ran through on long distance journeys between different railway companies tended to be painted in a livery that matched that of their parent companies, and their parent companies thus tended to synchronise their carriage liveries somewhat; so East Coast Joint Stock was always varnished teak (even though the North Eastern Railway's own internal carriages were painted dark red); and the LNWR and Caledonian railways had very similar carriage liveries, no doubt on purpose. Thus, it would be reasonable, on the basis of that research, to use the Midland Railway colour for the NBR sleepers that ran through to the Midland.
My apologies, incidentally, for not having had the chance to get to this topic sooner; I am slowly ploughing through the large backlog of very helpful contributions in amongst my own Christmas project which this year comprises enhancing the GWR offerings. Thank you again for your work on this: it is much appreciated.