I'm going to throw something out there; I have no idea of it's correct or if it just worked for me but it'll give you something (else) to think about .....
I passed my MMI at the end of 2014. I was lucky in-so-much-as, I had many examples for every question, having been in the army for 24 years.
Whilst preparing for the MMI, I spent many hours reading this forum and taking note of other applicants experiences, thoughts and outcomes. What I did see often were comments along the lines of "They must've been looking for a key word, they kept asking me the same thing again and again. I tried lots of different answers that I thought they were looking for but I still failed. Obviously I didn't give them the key word they were after."
Two things; I didn't buy-in to the theory that they would help you during the interview, that would keep asking to tease the right answer out of you. Also, if you are concise yet complete with your answers, keep it truthful but also bear in mind what they are looking for (a candidate suitable to train/become a train driver!), you shouldn't leave them with any gaps to be filled. Yeah, I know, easier said than done!
Anyway, here's my (controversial?) theory! I believe that the reason they repeatedly ask you a question isn't to tease another/better response out of you but to see how strong a character you are, to see if you can be easily swayed from your original thoughts/answer, to see if you are the confident type that 'has courage in your convictions'.
Following my MMI I was/am even more convinced that this was/is the case. My 'emergency situation' answer was based on being first at the scene of a really bad head-on RTA, as in, one car totally wrapped around a tree with the driver trapped and crushed inside; the other driver had broken bones pushed through his trousers and his big BMW was about the same length as a Nissan Micra due to the impact.
I explained that I used a bit of triage, told the next on scene to call the emergency services, sent a couple of people to treat the driver wrapped around the tree as for shock and I stayed with 'legs', trying to reassure him and keep him conscious etc. despite his chances looking pretty slim.
Back in the room, the interviewer listened as he busily made notes and/or ticked boxes. When I'd completed my answer, obviously using STAR, he then asked me how I felt at the time and afterwards. I answered honestly, "I felt nothing, not sad, not upset, not 'proud', just nothing. It was a kind of 'autopilot' thing, probably because of my military training and because at that point in my life, I'd seen and been involved with much worse."
Effectively, the interviewer asked the same question three times! And three times he got the same answer!! Finally I did say to him "I'm sorry if you're looking for something else, if you want me to tell something about how I felt, but I've told you everything already. I really don't have anything else to add." I then made a joke about how my ex-wife also told me I was heartless but hey, that's just me! Thankfully, he saw the funny side of it.
So to reiterate my point, maybe, just maybe some questions are asked to test your resolve, especially if the same question is being thrown at you again and again. Maybe the interviewer expected me to have a had a breakdown or slip into shock once the adrenaline (of the accident) had subsided but that wasn't the case so why should I say any different? It was a gamble and there was a chance that he was trying to help me 'tick a box' but personally, I really don't think so.
Finally my disclaimer! This was/is just my opinion! I believe it worked for me. You have to decide whether to follow or ignore this advice. I'm not responsible if it doesn't work for you or if I'm wrong and I was just lucky on the day.
Good luck!