Dixieland Jazz was a popular form of Jazz that originated in New Orleans at the beginning of the last century. When the Saints Go Marching In is a popular Dixieland song.
The following example is a basic arrangement of the song that combines an alternating thumb bass line with the melody line.
The next version of When the Saints Go Marching In uses a variation on the alternating thumb bass line for each chord as well as playing more notes within each chord.