I agree it'd be heartless to prosecute or sue a switch-thrower who was acting in good faith, but the family of someone killed often don't have a ton of sympathy.

http://www.cprinstructor.com/DC-GS.htm

Using DC as an example, I don't think that tampering with railroad equipment counts as "in good faith, rendering emergency medical care or assistance at the scene of an accident or other emergency" and it only covers against civil damages: basically it reduces private claims of negligence or liability when you did your best to stabilize an injured person. It gets into shaky ground when the person is not yet injured, and they become injured because of your actions. It also doesn't prevent the government from trying you for manslaughter.

It's definitely a messed up situation though, ideally we'd have further laws reducing the bystander effect and encouraging people to do whatever's possible to help. Often we see that people already do, though, and fortunately(?) the situations are often far less clear cut and diabolical than the Trolley Problem.