It doesn't account for the likelihood that my actions fail to save anyone and simply increase the death toll either, which would be a paralysing fear for many people in real life.

This particular scenario seems an odd one because I simply can't conceive a situation in which I could ever be confident a falling fat man would stop a moving train. My instinct would surely be to trust everything I'd previously learned about the laws of physics.

Maybe! It depends on the circumstances... http://en.wikipedia.org/wiki/Milgram_experiment

There's also the fact that you would be making a complete guess that the man was large enough to stop the trolley in real life, and if he wasn't you would simply be adding another victim. Besides, a man large enough to stop a trolley would be really hard to push.

Exactly. There's no way to know if the action of pushing the fat man onto the tracks would actually stop the train. Worst case scenario, you might end up killing one extra person.

In the 'switching the track' example, on the other hand, the worst case scenario is that your actions have no effect on the outcome and no additional people die than would have died otherwise.

Why not just ask Chip or Tyrone whether they feel their lives are worth losing to save a hundred others?

If someone asked me that I'd push them over the edge!

This disconnect between what someone says and does when polled on questions involving race is called the 'Bradley Effect': http://en.wikipedia.org/wiki/Bradley_effect