Editing Harrow-on-the-Hill signal cabin

Harrow-on-the-Hill (prefix JB) is a signal cabin located above the [[Harrow-on-the-Hill|station of the same name]] on the [[Metropolitan line]] of [[London Underground]]’s Sub-Surface Railway (SSR). Opened in 1890, re-sited in 1948 and extended in 1960, it is the largest operational signal cabin on London Underground. It is scheduled for closure by 2023 as part of the [[Four Lines Modernisation]] (4LM) re-signalling programme.

The first signal box (prefix C) on this site opened in 1880 with the northward extension of the [[Metropolitan Railway]] (MR) from [[Willesden Green]] to Harrow.<ref>Jackson, Alan (1986). London's Metropolitan Railway. David & Charles. <nowiki>ISBN 0-7153-8839-8</nowiki>.</ref> Harrow at this time was a comparatively simple two track layout reflected in a 16-lever frame. It was situated at the ‘country’ end of the platforms and was in much the same style as later surviving MR signal boxes at Chorleywood and Chesham both dating from 1889.

The opening of the MR’s Uxbridge Branch and the creation of the [[Metropolitan and Great Central Joint Railway]] led to substantial increases in traffic necessitating the widening of the by then renamed Harrow-on-the-Hill station in 1908. This doubled the formation in the immediate area to four tracks with the signal box suitably re-equipped on site to house 70 levers. Further expansion of the Harrow-on-the-Hill station layout led to the splitting of control between Harrow South signal box (32 levers) in 1901 and Harrow North signal box (27 levers) in 1924 such was the proliferation of tracks in the area.

In 1933, the MR was incorporated into the [[London Passenger Transport Board]] (LPTB) along with [[Underground Electric Railways of London]] to form the unified entity of London Underground under the umbrella organisation of [[London Transport]] (LT).<ref>LPTB submission to Parliament, Beard, Tony (2002). "The Post War Period". By Tube Beyond Edgware. Capital Transport. p. 118. <nowiki>ISBN 1-85414-246-1</nowiki>.</ref> Between 1935 and 1940, LT embarked on an investment package known as the [[New Works Programme]] aiming to standardize the operational infrastructure of the formerly independent Underground lines. This programme, heavily delayed by WW2 also saw the widespread re-building of stations to cater for the increase in passenger numbers notably in suburbia. Harrow-on-the-Hill station was rebuilt into its present form in 1939<ref>Horne, Mike (2003). The Metropolitan Line. Capital Transport.</ref> with an elevated ticket hall over three island platforms flanked by six tracks. The original MR signal boxes remained in use until 1948 when the current signal cabin opened at the top of the station complex.

The new signal cabin, prefixed JB, fully opened on 18 April 1948 and was equipped with a Westinghouse ‘N style’ electrically interlocked miniature lever frame. The control areas were split into two distinct categories. Harrow North oversaw the junction of the same name, connections to/from the goods yard and all associated points and signals north of the station area. Meanwhile, Harrow South oversaw the junction of the same name and all associated points and signals south of the station area.

Unlike more conventionally designed lever frames, majority of the 95 levers on the ‘primary frame’ were classified as ‘route levers’ each controlling a combination of signal and point slots on ‘subsidiary frames’ at Harrow North and Harrow South Interlocking Machine Rooms (IMR). The IMR subsidiary frames were moved remotely by the setting of route levers by compressed air in response to actions on the primary frame by the signaller. In emergencies, the air supply on IMR subsidiary frames could be isolated if manual working was required.

This pioneering arrangement was designed to aid the throughput of trains in what was now an extensive web of lines and goods sidings handling an intensive service of local stopping, limited stop and express passenger services as well as a variety of fitted and un-fitted freight trains. With the combination of the primary frame and the two IMR subsidiary frames, there were 172 levers directly under the control of Harrow-on-the-Hill signal cabin encompassing a geographic area of less than a mile.

Major layout changes followed in the 1960's with the quadrupling of the Metropolitan line between Harrow North Junction and Moor Park in four stages between 1960 and 1962. The cabin subsequently gained control of additional IMR's at Pinner (prefix JD) and Northwood (prefix JF). A push button panel was added above the primary frame with the diagram extended accordingly showing the new areas of control.

Ironically, the very traffic that quadrupling had been envisaged to cater for began to spiral into decline. This followed the cessation of Metropolitan line services to Aylesbury following electrification to Amersham in 1960, closure of most of the Great Central Main Line by 1969 and the sharp decline in goods traffic due to the rapid growth of haulage on the motorway network. Following the closure of remaining goods yards at Harrow, Pinner and Northwood, heavy rationalisation of the track layout occurred in the Harrow North area. By 1992, only a fraction of what the frame had originally been equipped for remained in situ. It is in this state that frame mostly operates in today.

Harrow-on-the-Hill signal cabin is due to be closed as part of the 4LM programme. The combined signalling of the entire SSR made up of the Circle, Hammersmith & City, District and Metropolitan lines will transfer in phases to [[Hammersmith Service Control Centre]] (SCC) fulfilling the ambition of signal concentration and automation that dates back to the 1960’s. 4LM will also see the roll out of [[Communications Based Train Control]] (CBTC) by means of [[Thales]]’ SelTrac which will do away with most auxiliary colour light signals. Trackside signals will however remain in ‘interoperable’ areas where non-CBTC fitted rolling stock operate over the SSR in this case being [[Chiltern Railways]] services over the Main Line between Harrow South Junction and Mantles Wood Crossing just beyond [[Amersham]]. 4LM has been split into 14 Signal Migration Areas (SMA) of which Harrow-on-the-Hill signal cabin falls within SMA 9 between Preston Road, West Harrow and Moor Park, due to go live in February 2023.<ref>"Underground News". London Underground Railway Society. 706: 572. October 2020.</ref>