If you sign up to a job which includes Boxing Day work you should know you will need to get there without trains surely? And before people go off on one I have signed up to a job which dosnt include Boxing Day work.
I think the actual reality of the situation "on the ground" is that most people find a way to work around the situation. Thankfully we live in a much more flexible society as far as work goes than was the case 50 years ago, not to mention the fact that whatever some people say, we have far more leisure time (This weekend I got an automated e-mail from a business I am working with saying they are more or less shutting down from now until January 7).
Faced with no trains on the 26th, what most people do who rely on trains and perhaps have gone away a long distance to be with family is take another day off (Thursday 27th), and in that case probably Friday too, and maybe for example agree to work over New Year instead if need be.
At the same time the competing force against this is that we are also a much more mobile society than 50 years ago, so many people do want to be able to get around on the 26th, whatever their motivation is for that doesn't matter.
IMHO I can't see the Christmas Day situation changing at all, but it seems there are more train services running now on December 26 than some years ago (I think), and this trend will probably continue to creep in that direction, as I can't see it going the opposite way.