Now that you have distinguished yourself morally from people who put their vintage cars ahead of a child's life, how about treating yourself and your partner to dinner at your favorite restaurant? But wait. The money you will spend at the restaurant could also help save the lives of children overseas! True, you weren't planning to blow $200 tonight, but if you were to give up dining out just for one month, you would easily save that amount. And what is one month's dining out, compared to a child's life? There's the rub. Since there are a lot of desperately needy children in the world, there will always be another child whose life you could save for another $200. Are you therefore obliged to keep giving until you have nothing left? At what point can you stop?
Hypothetical examples can easily become farcical. Consider Bob. How far past losing the Bugatti should he go? Imagine that Bob had got his foot stuck in the track of the siding, and if he diverted the train, then before it rammed the car it would also amputate his big toe. Should he still throw the switch? What if it would amputate his foot? His entire leg?
As absurd as the Bugatti scenario gets when pushed to extremes, the point it raises is a serious one: only when the sacrifices become very significant indeed would most people be prepared to say that Bob does nothing wrong when he decides not to throw the switch. Of course, most people could be wrong; we can't decide moral issues by taking opinion polls. But consider for yourself the level of sacrifice that you would demand of Bob, and then think about how much money you would have to give away in order to make a sacrifice that is roughly equal to that. It's almost certainly much, much more than $200. For most middle-class Americans, it could easily be more like $200,000.