The F Stock was built in 1920 and 1921 for the Metropolitan District Railway (later the London Underground District Line). 100 steel-bodied cars were built: 40 driving motors, 12 control trailers and 48 trailers. They had non-standard equipment that precluded multiple-unit operation with any other type of train and had significantly more powerful motors than earlier rolling stock, leading to some initial operating difficulties. As built, the stark interiors proved unpopular and were soon refurbished. The front ends of driving cabs had two distinctive oval windows separated by the centre door.