Quite interesting dilemma, I must confess.
It's a pretty straight forward answer for me. I'll let the train decide for itself.
I can't divert it to the single man (be it my friend, family or just anyone) because I might not know his situation, he man be the only child left in his family and probably has a bright future ahead.
So, it shouldn't be in my powers to decide who dies or who stays. I'll just watch the action while screaming and hoping they don't die after been hit by the train :)
It really is a difficult decision. And whatever decision you take or don't take, you will still be living with that burden.