What was the first ever London Underground line to be built?

The story behind the answer

Metropolitan Line is correct because the Metropolitan Railway opened in 1863 between Paddington (Bishop’s Road) and Farringdon—the world’s first underground railway. Built using “cut-and-cover” construction and worked by steam locomotives, it set the template for urban mass transit long before deep-level electric “tube” lines existed.

Context to remember: the Circle Line came later by linking the Metropolitan and District Railways into a loop; the Bakerloo Line opened in 1906 as a deep-level tube; and the Victoria Line is much newer (1968). Memory tip: “M is for Mother”—the Metropolitan was the mother of the Tube, pioneering the network that London (and subways worldwide) would follow.