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.
On your busiest commuter services I've seen many passengers looking seriously uncomfortable on you newest trains, and some even looking distressed. Bearing in mind the risk of a cardiovascular attack escalates meaningfully and rapidly from 24C it seems odd to me that you think 24C is an appropriate target. Can I suggest you re-introduce opening windows so that your customers get an option when you get it wrong?