More at http://philosophytalk.org/shows/when-it-wrong-save-life-lessons-trolley-problem.

A trolley is approaching a track junction, and you happen to be standing by the switch. If you do nothing, the trolley will kill a number of innocent children playing on the tracks. If you throw the switch, it will kill only one fat man, who is sleeping on the tracks. The so-called Trolley Problem sheds light on many claims in moral philosophy: utilitarian positions (doing what's best for the greatest number), the difference between doing and letting happen (being more obliged to not cause harm than to prevent harm), and issues of "collateral damage" (killing one person to save others). John and Ken ride the trolley with Thomas Cathcart, author of "The Trolley Problem, or Would You Throw the Fat Guy Off the Bridge: A Philosophical Conundrum."