Is it better to intervene to cause the death of a few rather than to let events play out that will cause the death of many? ... Personally, assuming that the situation is simply 'allow 40 to die' or 'kill 5 instead', the best outcome is to kill the 5.

Okay, instead of a runaway train, let's change the situation to a long-term financial depression. Greg is now the leader of a nation of people who are suffering through a severe financial crisis which is causing widespread poverty and hunger. A small group of people in this nation has a disproportionately high number of financiers and bankers in it. Should Greg sacrifice this much smaller subset of people in order to allow the much larger group of people in his nation to work their way out of the depression without interference from these financiers?

Yes, I just pulled a Godwin. The depression is the Great Depression. The nation is Germany. The smaller subset of people who are disproportionately bankers and financiers are Jews, and the sacrifice is concentration camps.

The thing is, we can frequently know when we are doing something wrong, but we frequently cannot know what the specific outcomes will be. We just aren't that smart.

In fact, we are sometimes so dumb that other people can't believe we really did something so awful just because we didn't know better, and then attribute our actions to malice. Yet, throughout the Nuremberg Trials we were repeatedly told that no one believed themselves to be acting out of malice. Yes, they knew what they were doing was wrong, but they believed the particular outcome they predicted would make it right.