Why heat the trains to 23C-24C in winter?

Who decides what temperature trains should be heated to? On the old rolling stock it is possible to open the window when a carriage is inappropriately heated, not so the new rolling stock. A central heating survey suggests that on average we heat our home to between 20-21C, so why do trains need to be hotter? Many passengers are wearing outdoor clothing so surely heating the trains to 23-24C is simply wrong as well as wasteful.