Either the railway is like other businesses or not. If it is, then it would not run half the trains it does, as they are probably not near the break-even point. Also, a huge number of businesses do NOT operate on Boxing Day.
There are alternatives for travel on Boxing Day - we have already established that. If TOC's chose not to operate, well, that is their prerogative as private companies. If the Government wants trains to run on Boxing Day, well, they can increase the subsidy accordingly, or the TOC's can charge an appropriate fare. That's what happens in business.
The bottom line is that TOC's are not looking to operate on Boxing Day. It's not staff are refusing to, they are not being asked to. What is so difficult about that, that some people can't grasp the concept?