I'd say that the blood from either outcome is on the hands of the person who set up the scenario.
Also, I'd have thought that the innocent bystander would also be tied to the tracks, such that it was "these multiple people tied to the tracks die" or "this one person tied to the tracks dies". If the bystander isn't tied to the tracks, then he can get the heck out of the way and the choice is easy and bloodless.