Consider the following scenario:
You are in a tower at a rail yard where you can control one switch. A train full of passengers is approaching the fork in the tracks with that switch. You cannot leave the tower or call anyone until the train passes.
On one track, a person is tied to the rails. The other track is damaged. If you do nothing, the train will continue onto the damaged track and derail. Many of the passengers and crew will be killed as a result.
If you switch the train onto the other track, it will pass safely onto its destination, but it will kill the person tied to the tracks.
It's your decision . . .
[Additional details: (1) Once the train passes the fork in the tracks, it can’t be stopped until it hits the person who’s tied up or derails. (2) Switching the tracks isn't actually your job; you just happen to be in the tower because of a set of random circumstances. (3) The person on the tracks was already tied to them when you got there.]