From the longer Wikipedia page [1].
Metro-land (or Metroland) is a name given to the suburban areas that were built to the north west of London in the counties of Buckinghamshire, Hertfordshire and Middlesex in the early part of the 20th century that were served by the Metropolitan Railway (the Met), an example of Ribbon development. The Railway was in a privileged position allowing it to retain surplus land; from 1919 this was developed for housing by the nominally independent Metropolitan Railway Country Estates Limited. The term "Metro-land" was coined by the Met's marketing department in 1915 when the Guide to the Extension Line became the Metro-land guide. It promoted a dream of a modern home in beautiful countryside with a fast railway service to central London until the Met was absorbed into the London Passenger Transport Board in 1933.
The London Transport Museum page is [2], and there is various film and other coverage.