I am not sure why you'd turn the air con up or down. It should regulate the temperature and heat, or cool, as required to get it to around 21 degrees, give or take.
There really shouldn't be any form of control for anyone except the driver, who should be able to control things if s/he is too hot or cold. You can't exactly ask every passenger on the train what temperature they'd like, unless you give every single person their own vent and a control from 16-30 degrees!
I think one problem with air conditioning isn't just the obvious (it's broken) but how hard it will have to work to get the temperature down if a train suddenly gets very busy. Humans radiate loads of heat, so it is always going to be hard to get the temperature down quickly in these situations - and many passengers rushing for a train will be hot when they board, but cool down after 5-10 minutes by themselves.
Of course, if the air conditioning system isn't powerful enough in the first place then you're buggered anyway!