Well, my answer wouldn't change for any of those additions. Assuming both were the same distance from me, I would still save the younger guy no matter when we were going to die. I'd still save the younger guy even if I knew a bomb was on the way.
Plus I hate old people, so even if there was only the old guy on the railroad tracks I would probably just push him down to ensure he couldn't get away.
Heh, j/k
