Wow! I was going to say that there was no way I could kill someone but under these circustances you wouldn't really have a choice. I think I would redirect to the track with only three people and then hope that some greater power would intervene.

oh! the Trolley problem...its a common test/experiment in ethics. They say and I quote

"Such dilemmas are intended to test the limits and weakness of the major moral theories addressing the issues set forth by ethics and morality in general."
http://en.wikipedia.org/wiki/Trolley_problem

And I would really like to be able to put this into my own words but the quote below is the best way of answering the question, if there is an answer NB - I haven't changed the wording but the same situation is explained...
"The utilitarian will say, “I switch the trolley to the new track, because it’s better to kill one person than to kill five.” The non-utilitarian will say, “I leave the trolley to kill the five, because at least then I am not morally at fault.” The main ethical question is: “Is there a difference between killing a person and letting a person die?”
http://www.overthinkingit.com/2008/07/24/the-philosophy-of-batman/

A utilitarian is someone who believes that the morally correct course of action is the one that produces benefit for the greatest number of people.
http://www.google.com.au/dictionary?aq=f&langpair=en|en&hl=en&q=utilitarian

So who am I? I still don't know, I've never had to make the decision and I really believe it's one that cannot be answered until its required by action.