What did Judith Jarvis Thomson’s work on the trolley problem involve?

Judith Jarvis Thomson introduced variants of the trolley problem that challenged Philippa Foot’s duty-based analysis, exploring scenarios where moral intuitions differ, such as the “fat man” on the footbridge. In that situation, two bystanders witness the emergency from a footbridge over the track leading to the five workers. One of them, understanding that the trolley can be stopped only if a heavy object is thrown in its path, pushes the other, a fat man, off the bridge and onto the track, thereby halting the trolley and saving the five workers—but of course killing the fat man.