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The London Underground (often shortened to the Underground) is a rapid transit system in theUnited Kingdom, serving a large part of Greater London and some parts of Buckinghamshire,Hertfordshire and Essex. It incorporates the oldest section of underground railway in the world, which opened in 1863 and now forms part of the Circle, Hammersmith & City, and Metropolitanlines;^[3] and the first line to operate electric trains, in 1890, now part of the Northern line.^[4]

The Underground system is also colloquially called the Tube. As commonly used today both by Londoners and in most official publicity, this term embraces the entire system.^[5] It originally applied only to the deep-level lines with trains of a smaller and more circular cross-section, and served to distinguish them from the sub-surface "cut-and-cover" lines that were built first and originally usedsteam locomotives.