As a parent, not only would I not hit the switch, I'd blame Congress for the passenger deaths for not funding modernization of the switching infrastructure.
If blame is a meaningful thing to assign, I'm going to assign at least some of it to you for not watching your son, and quite a lot of it to you for not flipping the switch.
I much prefer a world where you are sad and your son is dead, to a world where hundreds of people are dead, all their families are sad, and your son is alive.
(That said, it's at least somewhat plausible that if I was in that situation, I would also make the wrong decision. That doesn't make it right.)
I'm confident most people would do the 'wrong' thing. A train-load of strangers is abstract, and it is difficult to comprehend. I think it is like in Schindler's List. 6 million Jews - who can grasp that? It is a number that makes no sense, a horror you cannot attach a face to. But we all are immediately choked up over the little girl in red.
I much prefer a world where you are sad and your son is dead, to a world where hundreds of people are dead, all their families are sad, and your son is alive.
(That said, it's at least somewhat plausible that if I was in that situation, I would also make the wrong decision. That doesn't make it right.)