A very good read.  Long, but worth working through.  The bit about philosopher-soldiers is both fascinating and scary.

...an out-of-control trolley—or train—is hurtling towards five people on the track, who face certain death. You are nearby and, by turning a switch, could send the trolley onto a spur and save their lives. But one man is chained to the spur and would be killed if the trolley is diverted. Should you flick the switch?

...the same trolley is about to kill five people. This time, you are on a footbridge overlooking the track, next to a fat man. (The Fat Man is now sometimes described as a large gentleman. But fat or large, the fact of his corpulence is essential.) If you were to push him off the bridge onto the track his bulk would stop the trolley and save the lives of those five people—but kill him. Do you push him?

via Prospect Magazine.