>Probably it's useless to say that, from my direct experience as well from the results I know in other companies, the best programmers were selected using this kind of questions and not language very specific questions (as I have seen just too many times). Obviously, such questions must be combined with language/prgramming questions. The real advantage is that the solutions to such problems cannot be learned from books, while language/programming answers can be...

Actually, there's nothing wrong with learning from books, but then you're right about questions: if book knowledge is a set of standard answers to standard questions, you won't get the best coder asking just standard questions. You actually want to know how does the candidate apply the (book or practical) knowledge to a real life situation. It's hard to simulate such a situation, but then that's why we are in search of questions like that.

Though, I still don't like your train question - there are undefined defaults, like what does "train meet" mean? When the drivers can see the whites of each other's eyes ? :)

Thinking it over, the question is proper for this sort of tests - you usually don't get requests which were thought out thoroughly; sometimes the best time saver is your ability to point out the impossiblity of implementation right away, before anyone starts any coding.