I told this story some time ago, but think it's worth reiterating.
In the early 1970s I was commuting daily between Sittingbourne and London. On an evening rush hour train from Victoria a couple and approx. 4 year old son were sitting opposite me, in an open carriage with door on each side, doors which could be simply opened from inside the carriage at any point in the journey i.e. what is now referred to as slam-door stock.
The child is allowed by his parents to go and stand by the door and start fiddling with the door handle as the train leaves Victoria, bound for Bromley South first stop. The person sat next to me, beside the door, decides to do the English thing and pretend they haven't noticed anything. I got angry and demanded that the parents restrain their child whereupon I got the response from the father 'if he falls out I shall sue the railway company.' I then saw red and said I would pull the emergency cord if they didn't remove their child - I think I flashed my London Transport staff pass at the same time, in the hope that they might think I could cause them problems (after all, BR had been in existence since 1948, which seemed to have escaped their attention, so they couldn't be expected to know the difference). It did the trick, though, and they remained seated and silent until getting off at Bromley. I like to think I gave an appropriate glare as they got off! A very likely fatality saved I felt, but it certainly made me think how stupid it was that such an occurrence was possible.