trolley problem

An ethics thought experiment, about which choice in the dilemma is more ethical.

From the Wikipedia article on trolley problem:
You see a runaway trolley moving toward five tied-up (or otherwise incapacitated) people lying on the tracks. You are standing next to a lever that controls a switch. If you pull the lever, the trolley will be redirected onto a side track, and the five people on the main track will be saved. However, there is a single person lying on the side track. You have two options:

1. Do nothing and allow the trolley to kill the five people on the main track.
2. Pull the lever, diverting the trolley onto the side track where it will kill one person.

Which is the more ethical option?

While the subject of jokes for some years, it blew up on Japanese Twitter after a high school teacher wrote about how he had once presented the problem in class, and one pupil responded by saying he wouldn't pull it under the argument of being a bystander, and that he wasn't responsible for the deaths of those five people if he didn't get involved, but he WOULD be responsible for the death of the one person if he did.

The image of the problem used in the tweet is taken from the book 論理的思考力を鍛える33の思考実験, depicting the problem with a minecart filled with coal rather than a trolley.