I've just listened to this for the first time on Youtube (Jimmie Driftwood version) Brought tears to my eyes.

As a Prison Visitor I went to see one of 'my' prisoners in hospital a few years ago. He was gravely ill and they'd brought him there from prison in an ambulance. He too 'had a long chain on' as he lay in his hospital bed. The chain was attached to the wrist of one of the two Officers who sat by his bed in turns day and night.
I felt it was inhuman and asked if they could not release him (he couldn't sit up and was barely conscious) but it was against the rules.
I went home and wept. He died a few days later.

I think I'm drawn to the explanation of the song which concerns the slaves who built the underground railway.