I think there is a legitimate out for the utilitarian logician here, based on the fact that this scenario will never obtain in reality. In the ivory tower, the utilitarian says that "It is better for five people to die than more than five" and this is logically equivalent to saying "In thought experiments, it is better to take an action which leads to five deaths than more than five deaths, ceteris paribus."

In the real world, we do not enjoy this kind of mathematical certainty. What if pulling the lever derails the train and hundreds are killed? What if the tunnel with forty workers collapses as the train kills the five in the other tunnel?

NotPhil makes this point, though I wonder if he will keep to the non-interventionist high road if there are no workers on the alternate track, and therefore a chance of saving everyone.

In the real world, uncertainty and values apply. Greg may or may not be willing to interfere with fate if there's a good chance it will save forty and put five at risk. He may also consider whether he knows and cares about any of the potential victims.

This also helps with the horrifying scenario of chopping up one healthy person to get organs which will save a number of sick people. Killing someone for their organs is exceedingly objectionable, and certainly not worth it to have a chance of saving two or three others, assuming the transplants go well.

But if a pandemic threatens with high probability to destroy the human race, and a misanthropic scientist is threatening to destroy the effective antidote he invented, it may be justified to call the snipers in.