The expansion of London's railways played a significant part in the demise of Cassiobury House. The incursion of the Metropolitan Railway into Hertfordshire reached Watford in 1926 with the opening of Watford tube station and the Metro-land building programme was at its peak. Developers such as William King & Co and Charles Brightman acquired valuable land around Cassiobury to lay out new streets and build new houses in the vicinity of the new station. The former site of Cassiobury House was eventually built over by a suburban housing development. In 1930, more land for the public park was purchased by Watford Borough Council. A set of grand castellated Tudor-style gates survived at the Rickmansworth Road entrance to the park for several decades, but were finally demolished in 1970 by Watford Borough Council to allow road widening along the A412 road.