“Tightrope” is a title that Bono mentioned as a working title for the Songs of Experience album.
In October 2015, Entertainment Weekly published an article which interviews Bono and gives a look ahead at Songs of Experience. Part of that article was a mention of a new song, “Tightrope”:
“We’re well into it,” Bono tells EW. “Edge just came up with a cracker the other day! It’s provisionally titled ‘Tightrope.‘”
As far as how the new material will sound, Bono says of that track, “It’s very epic. It’s very up. And, you know, that’s probably what the experience has taught us: to be fully in the moment and that fun is respected, and joy is not to be let go of.”
The song has not been mentioned since that interview.