Indeed they do - but no way of tracking where or which depot that train is at at the point of requiring service.
So do you give the job of tracking the defibs to some depot manager who passes on the defib info if/when that train gets seconded or transferred, possibly across country. Not only do you need to advise the company providing battery support, but the 999 operator.
Not exactly straight forward
With modern rolling stock the train maintenance teams know exactly where every coach is at any time, all rolling stock is tracked using GPS, can feed back live data or the data is downloaded as it enters a depot. Maintenance records will hold serial numbers of all key components when last inspected, due for replacement .............. so an AED not a problem.
Not forgetting the good old fashioned planned preventative maintenance using visual inspection