By the mid-nineteenth century, the first major suburban areas were springing up around London as the city (then the largest in the world) became more overcrowded and unsanitary. A major catalyst in the growth in urban sprawl came from the opening of the Metropolitan Railway in the 1860s. The line joined the capital's financial heart in the City to what were to become the suburbs of Middlesex. Harrow was reached in 1880, and the line eventually extended as far as in Buckinghamshire, more than from Baker Street and the centre of London.