Would You Kill the Fat Man?

Here’s the scenario. Walking across a bridge over a railroad one day, you notice that five people are tied to the tracks below. Worse, you also spot a speeding train approaching, with no sign of slowing down—it’s sure to plow through the five people, killing them. Suddenly you see the only possible way to save them: an exceptionally large man—large enough to derail an oncoming train, it just so happens—is leaning on the bridge’s railing above the tracks, resting. Now’s your chance: do you push the man over the railing, killing him, but saving the five people tied to the tracks? Or do you refrain from pushing him, thereby sparing his life but effectively allowing the five below to die?