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 kill

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.