Was there ever a situation where you had to make a difficult decision, and there was two or maybe three possible options, each better than the next, and all within the parameters the interviewers are looking for?
Now, say you chose the best solution, which for arguements sake was option C, and that was it, job done, no room for improvement, or at least no 'obvious' room for improvement.
Would it not be an option then when asked this question you said you used plan B, which still had the desired result, but then left you with the better alternative in C, (the one you actually used) which you realised through 'hindsight', and therefor you'd do the next time?
I'm not saying this is what you should do, only using it as an example of looking at it from a slightly different perspective.
Hope that makes sense.
Interesting direction to follow, if you are set a task and the desired outcome is say for instance is to stop your train smoothly at the 4 car mark, what is the most important factor, to stop at the 4 car mark or stop smoothly? If you over run the 4 car mark by 2m say, the train is still on the platform and there is not a safety issue, if you stop at the 4 car mark and have to use a lot of brake at low speed you have actually stopped where required but not smoothly, now, what would be the correct thing to do? It is a personal choice.
The outcome of this scenario could actually be someone falls and injures themselves due to the heavy braking, so the smooth stop would have been the desired choice, but, if someone fell between the door and the plaform because of the curvature of the platform stopping at the 4 car mark with heavy braking would be the desired choice, this may seem irrerevant but if I asked this question at interview I would expect an answer containg both these factors and what would in the interviewees opinion be the preferred action. There is not a correct answer except really to ensure that whne driving a train you try your best not to be in this situation of making a choice on what is the best option due to an error.
A trainee asked me a question, I just cannot seem to get a smooth stop at the plaform with my DI no matter how hard I try, I asked him what options he had on ensuring he did stop smoothly, he said aim to stop just short and then release the brake at a slow pace to stop smoothly, I said try it, he came back and told me he had sussed it and now can stop smoothly.
This is why some sort of questions and certain aptitude tests are used, the driver used his common sence to answer his own question with a solution if fact this is him using his aptitude is it not.
Questions at interview are not there to catch you out but to see what common sence and aptitude levels you have, bear that in mind when answering questions and you should do well