My gradfather's first job when he moved from the old country was digging old freight tunnels by hand digging out that
Chicago clay. He was a strong man but it nearly killed him. He was worked so hard he said he literally used to sweat blood sometimes.
Tunnel under construction in 1902. The freshly cut clay ahead of the concrete work shows clear knife marks.
Trackwork in a typical grand union where two tunnels intersected, photographed before 1906.
https://en.wikipedia.org/wiki/Chicago_Tunnel_Company