The first verse of "Folsom Prison Blues" ends with the line "But that train keeps a-rollin' on down to San Antone," and the choruses of "What Am I Doing Hangin' 'Round?" include the line "I should be ridin' on that train to San Antone." The second verse of "Folsom Prison Blues" ends with the line "When I hear that whistle blowin', I hang my head and cry," and "What Am I Doing Hangin' 'Round?" also mentions a train whistle and the effect that it has on the narrator: "And still I can't stop thinkin' when I hear some whistle cryin' // What am I doin' hangin' 'round?"
I don't know if these similarities are intentional or coincidental, but there is a resemblance between the two songs. For what it's worth, "Folsom Prison Blues" was written by Johnny Cash and recorded on 30 July 1955 (the live version was recorded at Folsom Prison on 13 January 1968), and "What Am I Doing Hangin' 'Round?" was written by Michael Martin Murphey and Owen Castleman and, according to the liner notes of the deluxe edition of Pisces, Aquarius, Capricorn & Jones Ltd., was recorded on 20 June, 5 September, "& other dates" in 1967.