Suppose that there were more owners of priceless vintage cars Carol, Dave, Emma, Fred and so on, down to Ziggy all in exactly the same situation as Bob, with their own siding and their own switch, all sacrificing the child in order to preserve their own cherished car. We do not excuse them because others were behaving no better. These readers seem to be acting at least as badly as Bob was acting when he chose to let the runaway train hurtle toward the unsuspecting child. Perhaps you should do it before reading further. There's the rub.
Hypothetical examples can easily become farcical.
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. 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. I accept that we are unlikely to see, in the near or even medium-term future, a world in which it is normal for wealthy Americans to give the bulk of their wealth to strangers. When it comes to praising or blaming people for what they do, we tend to use a standard that is relative to some conception of normal behavior. Nevertheless, they should be doing much more, and they are in no position to criticize Bob for failing to make the much greater sacrifice of his Bugatti.