"Good Samaritan laws in the United States are laws protecting those from blame by choosing to aid others who are injured or ill. This law is intended to reduce bystander's hesitation to assist, for fear of being prosecuted for unintentional injury or wrongful death of those they aid. "
This law's corollary is that bystanders who choose NOT to aid others who are injured or ill are also immune from prosecution.
The interpretation of this law vary by state, but with few exceptions, anyone rendering aid in good faith to another is immune from lawsuits in the U.S.
Back to the runaway train question -- maybe the question is not who you should save on the tracks, but those on the train. The train hitting three people would be more likely to derail, causing an accident that could kill many more lives. If you're just playing a numbers game, let the train hit the lone moron on the second track.