From Bob Riesman's I Feel So Good: The Life and Times of Big Bill Broonzy. University of Chicago Press, 2011, p. 226:

The Northwestern [University] concert [October 25, 1956] was noteworthy for several reasons. It marked a milestone for WFMT because it represented the first concert held in the Chicago area that the station taped for future broadcast. In the years to come, WFMT would record hundreds of concerts by folk and classical artists. In the case of the Northwestern show, some of the material was later released on an LP by Folkways, in conjunction with Verve. For years, Big Bill Broonzy and Pete Seeger in Concert was the only commercially available recording of Bill performing live for an American audience.... The recording also documented one of Bill's most-quoted observations. As he was introducing his version of the spiritual "This Train," he observed, "I like all songs,, you know, and some people call this a folk song. Well, all the songs that I ever heard in my life was folk songs. I never heard horses sing none of 'em yet."