I agree with NotPhil. If these 45 people are interchangeable, to Greg, then I don't see why he has to be there in the first place. He should probably call whoever is in charge, if only to cover his own ass and also to get some medical personnel down there. That said, actively choosing to kill the five people would be wrong. So would letting 40 people die, but there's no indication that those 40 people would all die at once if the train goes down that tunnel. I'm pretty confident that at least some of them have a good chance of survival. Plus, that way he can go grab the five dudes he could have killed to help any of those 40 people who may have survived the train. It's likely better to have 5 people who are not in shock helping injured people. Then again, if all 40 people would definitely die and Greg somehow knew that, then ok, pull the lever.
Pulling the lever or not pulling the lever are both actions. Either way he is killing people. The moment he realized he could shape the outcome he became a participant. He can either participate in killing many or a few. He can either kill people sooner or slightly later. For me the "later" part is a big thing. Time = opportunity.
True, but that Greg can't know how many people may die by not diverting the train was a big influence on my decision. I realize that I assumed that Greg was not The Boss, but I'll stick to my guns here and say that I think he should make a phone call, since people are going to be injured or killed either way and he can't actually see what happened from his current vantage.