The enemy is volitional in OP's example as well. Those people didn't get tied to the tracks by the wind.
The difference between the two scenarios is this: in mine, you are allowed to know about the people involved, and you can therefor JUDGE them. That's what makes the decision possible. You can recognize evil, and act to defeat it.
In OP's scenario, you're supposed to make a decision without knowing anything about the person who set this up, why he's doing what he's doing, or about the people tied to the track for that matter. You're supposed to make your decisions without JUDGING the people involved.
You're supposed to decide that diverting the train is right or wrong irrespective of who the people involved are, what they have done, why they're in this situation, etc.
That's what's fundamentally wrong about it: it divorces ethics from context, and expects people to have a moral code that doesn't require them to make value judgements about people, or do any kind of thinking, before they can apply it indiscriminately.