The trolley is heading for a circular portion of the track. The switch controls which way around the circle the trolley will go, either clockwise or counter clockwise; it is set so the trolley will go to the right - counter clockwise. One third of the way around on the right side a mother is stuck on the track; one third of the way around on the left side a car containing her infant son is stuck on the track. When the trolley strikes the car it will derail.
If the switch is not thrown, the trolley will go to the right, killing the mother first, and then hitting the car killing the infant. It will then be derailed. If the switch is thrown the trolley will go around to the left and hit the car where it will again be derailed, but now the mother will live. Either way, the infant will die, but if (and only if) the switch is thrown will the mother live.
Do you throw the switch?