I don’t know what the backstory is, but a group of people decided to have a little fun on BART – that’s short for Bay Area Rapid Transit, the San Francisco-and-surroundign-areas commuter train – and set up some swings. The action was captured in this Flickr set, which includes the photo below:

Looking at these photos reminded me of Accordion City’s late and lamented Secret Swing, which I didn’t know has its own entry in Wikipedia.