On January 10th 1863 London and the world's first underground railway, the Metropolitan Railway, opened to the general public. The line had seven stations and served 38,000 people on the first day of opening. Much of the railway was done using the "cut and cover" method of getting a lot of navvies to cut a really big trench in the ground and then cover it over.

What is most notable about this is, at the time, the technology to properly run a railway underground did not actually exist. The trains were steam engines which, as was known from running them through tunnels, created appalling amounts of smoke with nowhere to go. Holes were made to vent at least some of this above ground, but the stations were still choked with smoke. It wasn't until the advent of the electric engine that the underground become pleasant to use.

Image: By Unknown (illegible) (The Illustrated London News, Issue 1181, page 692) [Public domain or Public domain], via Wikimedia Commons