In reality, pretty much anybody can use a defibrillator.They have them at large stations, but AFAIK not on trains. Due to the safety culture there must be a trained operator to use them and like first aid it is expensive (in people's time mostly) to keep people trained up.
British Heart Foundation said:Defibrillators are very easy to use. Although they don’t all look the same, they all function in broadly the same way. The machine gives clear spoken instructions. You don't need training to use one.
https://www.bhf.org.uk/how-you-can-help/how-to-save-a-life/defibrillators/how-to-use-a-defibrillator
It tells you what you need to do when you open one.