I think the cost of that would be prohibitive.
What is in my view needed is that, with a few possible exceptions like the Far North Line and S&C where other , there is a plan, including one which will work in severe weather, for the evacuation of any stuck train without toilets, or with an interior temperature less than say 15 degrees Celsius, within one hour, and any other stranded train within three hours.
For situations where this would not be possible, the train must carry an adequate amount of emergency equipment, which might include some kind of emergency toilet facility (bag and box based ones exist for use at festivals etc), an adequate quantity of water (I would say a 500ml bottle per seat would be a good starting point for this) and an adequate quantity of orange plastic mountain survival bags (one per seat as a minimum).