Chords from one of toadfrog's links:

[E] Mary had a baby oh my Lord
[A] Mary had a baby oh my Lord
[G] Mary had a baby [B7] oh my Lord
[A] People keep a-comin' an' the [B7] train done gone

Verses variant from another toadfrog link: Mary had a baby boy, oh, Lord.

Commentary from the last of toadfrog's links: Known comments by Bruce Cockburn about this song, by date: Album notes (Christmas (1993): "This call-and-response song appears to have originated on the South Carolina coastal island of Saint Helena in the last century. The verses follow a fairly predictable pattern till you get to 'moving in the elements...'"

~S~