Which X&Y hidden track was originally planned for Johnny Cash to record with Chris Martin?

The X&Y hidden track originally planned for Johnny Cash to record with Chris Martin was “Til Kingdom Come.”

It appears after the album’s twelve principal songs as an unlisted extra track. On physical editions, it is identified by a plus sign (“+”) in the disc label and booklet rather than by its full title, which is why it is often called X&Y’s hidden track.

The collaboration was intended to pair Cash’s country voice with Martin’s songwriting and performance. Cash died before the recording could take place, so the planned duet never happened. Coldplay instead included the song on X&Y, released in 2005.

A common mix-up is confusing it with “Swallowed in the Sea,” “Twisted Logic,” or “A Message,” all of which are regular named tracks on the album. “Til Kingdom Come” is the extra song associated with the unfinished Cash collaboration.