It’s not a coincidence that the world first underground rapid transit line — the [Metropolitan Railway]([https://en.wikipedia.org/wiki/Metropolitan_Railway](https://en.wikipedia.org/wiki/Metropolitan_Railway)) — opened in London in 1863. Keep in mind these were coal-burning, steam-hauled trains that were being operated *underground*, and “the Met” was still a massive success. London was so congested that thousands of residents happily paid to ride the Metropolitan underground through the soot and steam and grime. Electrification wouldn’t occur until around 1900, when the Metropolitan began to experience competition for ridership from the new deep level tubes, which began service with electric traction from their opening. Long story short, extreme congestion and population density in London lead to the direct development of underground mass transit.
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