As for a justification for the test answers I made, the people in a public thoroughfare (train tracks or roads) have agreed to take chances. They also have the opportunity to hear, turn, and run away from a noisy item, so in theory, they are given a fair chance. So to allow the one to chance the problem minimizes the deaths.
As for the healthy person in the hospital, he did not accept the "risk" of being cut up upon entry for the benefit of others, so he should be left with his body. However, if in that culture he knew that a healthy person had a reasonable chance of having random organs removed upon walking into a hospital, then I would accept the killing of that person to help the others.
And I'd bet that Singer would laugh at the idea of "Natural Law."
But, the whole purpose of the column is to tweak our utilitarianism.
Neither 1 nor 3 are "permissable."
No one may legitimately act to cause the death of any one else unless the one being killed is the actual threat to the life of another. The actions in 1 and 3 are wrong, even if the intention is good.
However, both are exactly why "bioethics" was created in the '70's: so those in power (by fact of majority, judicial representation or by convincing the actual majority to be still and quiet because "that's not fair!" or "my story is sadder, more deserving, more heartwrenching than yours - besides, I cry louder and more convincingly than you can!") may decide who will be killed and who will not.
We might forgive (and make seem "permissable") the switch operator in number 1 because of the emergent nature of the act - because people are not perfect and omniscient, people make mistakes in the heat of the moment. We react out of compassion for the 5 that outweighs our concern for one.
The argument could even be made that not deciding might seem a decision.
But, these are immature and emotional rationalizations, not ethics: no matter how well intentioned, no matter how good the balance of the consequences might seem, it is always wrong to act to cause the death of a human being who is not a danger to life of other human beings.
(Besides - The lone man could be me or you or the one man on earth who could cure all forms of cancer, while the 5 could be Hitler and his cabinet - Or Peter Singer and his co-authors.)
The utilitarian would like us to think of people as interchangeable parts with planned obsolescence - we should feel the same way about the strangers on the other side of the world as we do about our own families, right? Isn't that the meaning of "equality?"
But they turn their final answer to the lowest common denominator:
Health care for everyone from the first dollar, but each person can only have the same amount total. (Tylenol for free, but no dialysis after 55 years old)
Public education, but a minimal standard so no one's left out (and no one is rewarded for excellence or punished for lack of ability or effort).
Public transportation, but only the elite can ride in SUV's without being derided for being selfish.
Since people are interchangeable, and we can always make new ones, why waste "limited resources" on someone who costs us too much, or who can't (yet or anymore) interact with us?