traditional materials like brick. Holden called them 'brick boxes with concrete lids'. Today, several of these Holden-designed stations are listed buildings, including the prototype Sudbury Town listed as Grade II*. On 4 July 1932 the District service from Acton Town to South Harrow was withdrawn and one in three Piccadilly trains extended from Hammersmith to South Harrow, the District continuing to run a shuttle from South Harrow to Uxbridge. On 18 December 1932 all four tracks to Northfields opened and from 9 January 1933 Piccadilly trains started to run to Northfields, continuing to Hounslow West from 13 March 1933. District trains continued to run through to Hounslow off-peak, with a shuttle from South Acton. London Passenger Transport Board, 1933 The UERL's ownership of the highly profitable London General Omnibus Company (LGOC) since 1912 had enabled the UERL group, through the pooling of revenues, to use profits from the bus company to subsidise the less profitable railways. However, competition from numerous small bus companies during the early 1920s eroded the profitability of the LGOC and had a negative impact on the profitability of the group. To protect the UERL group's income, its chairman Lord Ashfield lobbied the government for regulation of transport services in the London area. Starting in 1923, a series of legislative initiatives were made in this direction, with Ashfield and Labour London County Councillor (later MP and Minister of Transport) Herbert Morrison, at the forefront of debates as to the level of regulation and public control under which transport services should be brought. Ashfield aimed for regulation that would give the UERL group protection from competition and allow it to take control of the LCC's tram system; Morrison preferred full public ownership. After seven years of false starts, a bill was announced at the end of 1930 for the formation of the London Passenger Transport Board (LPTB), a public corporation that would take control of the UERL, the Metropolitan Railway and all bus and tram operators within an area designated as the London Passenger Transport Area. The Board was a compromise – public ownership but not full nationalisation – and came into existence on 1933. On this date, ownership of the assets of the District and the other Underground companies transferred to the LPTB. Legacy The railway became the District line of London Transport. From 23 October 1933 a Piccadilly line service replaced the Harrow to Uxbridge District