Back at the start of the 20th century, boring a tunnel took a lot of hard, manual labor - you still needed big, burly men to hack away at the rock. The BPL has three sketches of men at work digging a tunnel for a trolley line between downtown Boston and Maverick Square - what eventually became the Blue Line (the link will also take you to some photos).
At the end of the tunnel, men had to work to make it even longer. The tunnel was deep enough to require an airlock to ward off the bends.
The top sketch appears to be of men working with this "roof shield:" (source, click on "page"):
The $3.2-million tunnel opened in 1904 - the first underwater tunnel for a North American subway:
BPL images used under this Creative Commons license.