Complex sentence: The Tube is a colloquial name for the London Underground, London's Underground public transportation system, the underground railway has been described as "the tube" since 1890, when the first electric railway line running through deep-level tunnels was opened.
Split into simpler sentences: The Tube is a colloquial name for the London Underground, London's Underground public transportation system. The underground railway has been described as "the tube" since 1890 when the first electric railway line running through deep-level tunnels was opened.