When i started on the railways (30 June 1980), railway services did not run on either Christmas day and Boxing day, in my current area of employment they still dont.
So because there were no train services on Boxing Day in 1980, that doesn't mean that there shouldn't be services today.
Any business (including the railways), whether publicly or privately owned, has to respond to the needs of customers, and those who work in andrun the business should recognise that their customers' needs can change over time.
I think there is clearly a demand for some services to run on Boxing Day.
When I moved to London in 1991, the Boxing Day service on the Victoria Line was every 15 minutes and only between Brixton and Seven Sisters. Now it is a full Sunday service (albeit with a later start) over the whole line. Transport for Londo run a full Sunday bus service on Boxing Day, which certainly didn't happen 20 - 30 years ago. Transport for London are also considering the viability of running a bus service on Christmas Day.
No-one is saying that there should be a full service over the entire National Rail network on Boxing Day. But the growth in the number of Boxing Day sales, plus the many sporting events that take place, suggests to me that there would be some demand for services.
In addition, people who do have to work on Boxing Day should be compensated adequately, with enhanced rates of pay and additional time off. There were a series of Boxing Day strikes on the London Underground a few year ago, but now the unions have negotiated a deal which is satisfactory to them and their members.