Editing Ealing Broadway interlocking machine room

}}Ealing Broadway (prefix WP) is an Interlocking Machine Room (IMR) and former signal cabin close to the station of the same name on the London Underground District and Central lines. It is one of the last working examples of a miniature lever frame on UK railways, with no confirmed replacement having been descoped from the Four Lines Modernisation (4LM) re-signalling programme as of 2021.

The first of the three railways through Ealing Broadway opened on 6 April 1838 as part of Isambard Kingdom Brunel's Great Western Railway (GWR) between Taplow and London Paddington. Ealing station did not open until December of that year renamed Ealing Broadway in 1875.<ref>MacDermot, E T (1927). "History of the Great Western Railway". "'1'" (1 ed.). London: Great Western Railway.</ref>

District Railway (DR) services commenced from an adjacent yet separate station on 1 July 1879 as an extension leaving the Richmond branch at Turnham Green. The DR signal cabin (prefix WP) was fitted with a full-size McKenzie & Holland interlocked frame of 37 levers.<ref name= ":0">Horne, Mike (2020). [http://www.metadyne.co.uk/pdf_files/LTSB_new.pdf Inventory of Signal Cabins and Other Interlockings]: London Transport Railway</ref> Whilst the DR and GWR stations were operationally separate, a track connection between the two facilitated a short-lived DR service between Mansion House and Windsor (now Windsor & Eton Central) between 1883-85. Electrification of the DR had been ongoing since 1903, and the first electric trains reached Ealing Broadway in 1905. The original DR station building was later rebuilt into the surviving structure that can be seen at street level today.<ref>Connor, Piers (1993). "The District Looks West". "Going Green: The Story of the District Line". Harrow Weald: Capital Transport. pp. 14, 16. ISBN <bdi>1-85414-157-0</bdi>.</ref>

Central London Railway (CLR) services commenced from Ealing Broadway on 3 August 1920 as part of a joint venture (Ealing & Shepard's Bush Railway) with the GWR to link the Great Western Mainline to the Shepherd's Bush goods complex and West London Railway. In return, CLR services would be extended from Wood Lane to Ealing Broadway. A two-track terminus at Ealing Broadway for the CLR was constructed to GWR designs, the hallmarks of which can still be seen today. The CLR platforms were controlled from a dedicated lever frame (prefix L), which was co-located in the existing DR signal cabin, where the space constraints on this site were.<ref name=":0" />

Despite the DR and CLR having merged into Underground Electric Railways of London (UERL) by 1920, the DR continued to retain a separate station and ticket hall from the CLR, which was integrated into the GWR station. This somewhat disjointed layout would last until 1961, when most of the GWR station was demolished as part of an oversite development by British Railways.

UERL, along with the Metropolitan Railway, was merged into the unified entity of London Underground as part of the London Passenger Transport Board under the London Transport (LT) brand in 1933. For the Underground, this resulted in the mass standardisation of operating practices ranging from station signage and publicity to rolling stock and signalling. Whilst many of these ambitions were heavily delayed by the outbreak of WW2, the presentation of a unified transportation network remains one of the enduring legacies of the London Transport era.

On 30 November 1952, the last signal cabin opened at Ealing Broadway, replacing the 1879 DR signal cabin and the CLR lever frame.<ref name= ":0"/> Being a comparatively late LT signal cabin, it was equipped with a Westinghouse push button desk dispensing with earlier hand-worked power frames, which had dominated many of the post-war signal cabins equipped by Westinghouse on the London Underground. Signallers would set up routes via push buttons, which would then be enacted by electrically interlocked relays and carried out by the lever frame of the interlocking machine room (IMR) powered by compressed air. In the event of a failure, the air supply to the IMR lever frame could be isolated and worked manually in the case of Ealing Broadway, resembling the operation of an N-style power frame.

Implementing push button working would represent one of the final evolutionary phases of signal cabins on the London Underground. Push button desks and the development of automated programme machines in the 1960s would enable control of entire lines to be concentrated into a single facility, doing away with de-centralised signal cabins. Ealing Broadway was subsequently demoted to IMR status after 11 May 1974, with Earl's Court Control Room assuming control of the District line and remote supervision of the Central line passing to White City signal cabin (prefix CG).<ref name= ":0"/>

Re-signalling with Automatic Train Operation (ATO) meant that Ealing Broadway (Central line) gained a Signalling Equipment Room (SER) in the early 1990s located next to the IMR and former signal cabin. The link between the District and Central lines had been severed sometime before re-signalling on 17 May 1993, with control initially passing to a panel in the White City signal cabin (prefix CG). Wood Lane control room would not assume control of this area until 6 July 1999; such were the complexities of integrating this line with its numerous signal cabins into a single facility.<ref name=":0" />

The introduction of the S stock between 2010 and 2017 proved the catalyst for wholesale re-signalling of the Sub-Surface Railway. Bombardier, manufacturers of the S stock fleet, were also contracted to replace legacy fixed-block signalling with Distance To Go Radio (DTGR), a form of Automatic Train Operation (ATO). The re-signalling was known as the Sub-Surface Upgrade Programme (SSUG). However, Bombardier's inexperience in delivering signalling systems soon started to drag the project behind schedule. Faced with mounting costs and delays, London Underground terminated the contract in 2013 at an estimated cost of £900m and an estimated delay to completion of five years.<ref> "London Underground and Bombardier abandon Tube signalling contract". International Railway Journal. 3 January 2014.</ref>

Thales won the re-tendered contract for SSR re-signalling in 2015, renamed Four Lines Modernisation (4LM). Thales would introduce Communications Based Train Control (CBTC), a variant of SelTrac ATO. Migration to ATO was split into 14 Signal Migration Areas (SMA), of which Ealing Broadway IMR lay in SMA 11 concerning the railway between Chiswick Park and Ealing Broadway. In advance of 4LM re-signalling, control of the District line between Barons Court and Ealing Broadway was transferred to the South Kensington control room as part of the Piccadilly Interim Control Upgrade (PICU) which migrated areas of control away from Earl's Court control room as well as dispensed use of programme machines which were considered life expired.

However, owing to financial pressures within Transport for London (TFL) exacerbated by the COVID-19 pandemic, SMA 11 was descoped from 4LM in February 2021. Plans, including the Piccadilly line taking over the Ealing Broadway branch and eventual re-signalling to ATO, remain ambitions but, as of 2021, remain unfunded. As a result, Ealing Broadway IMR is destined to become one of the last working examples of a miniature lever frame on UK railways after completion of the funded remainder of 4LM, the sole other example being Maidstone East signal box in Kent operated by Network Rail.