If you choose not to decide, you still have made a choice. (I'm sure some people saw that coming).

Right/wrong from the perspective of Greg and right/wrong in the eyes of other people/society/the law might not always be the same thing. If Greg had not pulled the lever, looking back he might feel guilty for letting 40 people die, while other people will sympathise with him not being able to make a decision in time (and therefore doing nothing). Vice versa with pulling the lever - he might think it was justified, other people might call him a murderer. The tabloid press will pick a side, as it always does.

Hiroshima/Nagasaki is a famous example of this, too. It's made more complicated because not everyone had all the facts back then (no doubt people will be arguing about the details for a long time). I wonder if not knowing all of the consequences makes it easier or harder to decide. On the one hand, you can't know for sure which decision is right in advance. On the other hand, not knowing the consequences relieves you of some of the responsibility for your actions (future events are determined by chance). But that leaves you with a new dilemma: do you pick the action that has probability 0.1 of saving 100 lives, or probability 0.2 of saving 50? I feel like this hasn't really solved the problem, just put it into a different form.

Maybe Greg should have flipped a coin to choose what he would do. How about a coin with 1/9 chance of leaving the lever and 8/9 chance of pulling it? (I think I have got the numbers right)