Summer.
Alrighty. So two trains (A and B) are about to collide in the middle, killing 5 people tied to the tracks. However, you or the other person may redirect the other train to kill just one person.
However, if only one of you redirects a train (e.g. A) , the other train (B) will kill the 5 and the other person (person controlling A).
You cannot communicate with each other. You are controlling train A.
Will you choose to send it down the middle, killing 5 and potentially the other person, or redirect it, killing 1 and potentially sacrificing yourself (and, with the 5 that die in the middle as B hurtles past, meaning the 1 died in vain).