What was the first deep-level electric railway, now forming part of the London Underground?

The City and South London Railway was the first deep-level electric railway, now forming part of the London Underground.

It opened in 1890 between Stockwell and King William Street, using electric locomotives to haul passenger carriages through twin circular tunnels. The railway was built deep below the streets, which led to the enduring nickname “Tube.”

Its original northern terminus, King William Street, had operational limitations and was replaced by a new alignment toward Bank and Moorgate in 1900. The line was later extended and merged with other routes.

Today, most of the former City and South London Railway forms the southern section of the Northern line. It is sometimes confused with the Metropolitan Railway, which was earlier but ran mainly in cut-and-cover tunnels rather than deep-level tubes.