On November 4, 1890, the City and South London Railway opened, and it was the first deep level electric railway in the world, running in a pair of tunnels beneath the Thames between the City of London and Stockwell. The line had been promoted as a way to carry people who were then jammed into horse buses and steam trains above, and it had taken years to build and had run out of money more than once. The tunnels were narrow. Small electric locomotives pulled the carriages. There was no smoke.
The earlier underground line in London, opened in 1863, had used steam locomotives in shallow cuttings and the smoke had made it unpleasant, and the new method allowed a tunnel to be cut deep through the clay under the city without vents to the surface. The change was big. The engineers used a shield to bore the tunnels and lined them with iron rings, and the same method was used for the lines that followed, and the deep tunnels let the railways go under buildings and rivers without disturbing what stood above.
The line was short and the stations were few, and the first years brought more traffic than the company expected, and the tunnels were extended north and south over the following decades. The system grew. Other cities took up the method, and electric underground railways were built in Budapest and Paris and Berlin and New York before the First World War, and the tunnels under London grew into the network that carries millions of people a day.
The original tunnels are still in use, and the line is now part of the Northern line of the London Underground, and the name of the company has gone. The carriages of 1890 had no windows worth the name and the ride was rough, and passengers complained about the heat and the crowding within weeks of the opening. The complaints have not changed much. The idea of a city carried on railways under its streets began with a short line under the river.