The Metropolitan line in London is named after the Metropolitan Railway, which was the world's first underground passenger railway, opening in 1863. The name "Metropolitan" was chosen to reflect its purpose of serving the great "Metropolis" of London, connecting the city’s main-line railway termini with the financial district. Originally, the railway was an independent company that sought to expand its reach far beyond the city center, eventually stretching deep into the rural counties of Buckinghamshire and Hertfordshire—a region later famously branded as "Metro-land." This expansion was unique because the company acted as a real estate developer, building housing estates along its tracks to create its own customer base. When the various private underground lines were unified under the London Passenger Transport Board in 1933, the name was retained for the specific route that evolved into the modern "purple" line. Today, it remains the only London Underground line to cross the Greater London boundary, continuing the legacy of its ambitious "Metropolitan" origins as a bridge between the urban heart and the suburban fringe.