Probably two different issues but a connection. The problems at Kings Cross (and Paddington) were the result of over running engineering work and how that was managed - remember though that vast amounts of other engineering work took place on the 25th & 26th December which was completed on time. Lack of services on 26th December being an on going issue which the politicians and rail industry appear unwilling to tackle.
In the attic I have all the public timetable holiday supplements issued for the National Railway Timetable from its inception in 1974 until they ceased c2001. I will check these in the next day or so but other records to hand suggest that the last time trains operated throughout the network on the 26th was 1979. I am happy to delve into these supplements to clarify any other points on this topic.
A few points worth considering about the 26th:-
1) Regarding timetabling, no two Yuletide periods are going to be the same. By the time the 26th falls on a Friday again timetables and the publics travel needs will have changed, Thus, much more skill is needed to adapt the timetable than is needed for example the Late Summer Bank Holiday which is always the last Monday in August every year.
2) Lack of trains on the 26th has been a regular gripe of Allan Williams in Modern Railways for many years but he has focused on the lack of localised services on one of the busiest retail and sporting days of the year. As I believe Mike realises the lack of long distance trains is equally, if not more, important.
3) In recent years it has become the presumption that the Yuletide break is a one long holiday period extended up to a fortnight. This might be the case for some but many only have the 25th and 26th as holidays if they are lucky, as was the case for almost all in the not so recent past. It is only since 1977 (?) that New Years Day has again been a bank holiday in England & Wales. Much is made of seeing family etc at Xmas, well if you only have the 25th & 26th as holidays how are you going to see the family if the railway does not run.
4) From the 1970s until their demise in the 1990s I took a detailed interest in the relief trains passing through Sheffield. The Yuletide period being particularly fascinating. What was notable was how in the 1980s the number of relief trains, particularly late in the day, on the 24th declined. Similarly services started to thin out earlier in the evening than had once been the case. I suspect this was a consequence of withdrawal of trains on the 26th. Those who only had the 25th & 26th as holidays no longer being able to make journeys to relatives that they once could.
5) Certainly the lack of local bus services in many areas on the 26th is a consequence of trade unions demanding more and more for working then, thus making operation uneconomic. I believe similar has been the case on the railways. It is often presumed that those working on the 25th & 26th are earning a fortune. That might be the case on the railways but not elsewhere including retail were many earn the same whichever day they work. Locally we have a bakery whose staff are paid the same whichever days they are rostered in the year with the 25th & 26th being treated as normal days. How much have union demands influenced the situation that has developed on the 26th? Ironically there are many who would work on the 26th for normal money, taking the day as holiday at another time being much more beneficial to them.
6) It is apparent that formulating a special timetable is in the too difficult to do category these days. Just look at the nonsense we have in the Sheffield area were an almost normal weekday local service runs on bank holidays (including New Years Day). Consequently many early morning trains run empty or with single figure loadings. Back in BR days carefully planned reduced services would have operated. Thus, it is probably too much bother to formulate a timetable for the 26th which would reflect demand and jump through all the hoops. In the 1970s when trains ran on the 26th most early morning trains were cancelled with more running, including reliefs on NE-SW as the day progressed. There were even years such as 1977 when a completely separate inter-connecting timetable existed.
7) Long distance travel demand on the 26th this year would probably have been low with it being a Friday. However. if it had fallen on a Tuesday demand would have been far greater. Even so, had trains run then demand early on the 27th at Kings Cross would not have been so great and thus the situation would have been more manageable.
8) If you want an example to put the railway to shame take a look at Western Ferries who sail from Gourock to Dunoon (Scotland) 365 days a year http://www.western-ferries.co.uk/xmas_2014.pdf
They get no subsidy other than for concessionary reimbursements. There is an alternative, all be it lengthy road route, however the alternative subsidised Argyle Ferries, like all others in Scotland, do not sail on the 25th or 1st (Scottish equivalent to 26th).
9) Christmas Day, whenever it falls is a public holiday. The only other public holiday is Good Friday when trains run. The 26th is either a bank holiday or ordinary Saturday/Sunday depending when it falls. So, why do trains run on all other bank holidays, Saturdays & Sundays, but not the 26th.
10) National Express operate express coach services on the 26th. These have to be pre-booked and their drivers tell me they very rarely have empty seats other than no-shows and turn prospective passengers away.