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.