Amersham signal cabin

Amersham
JW
Overview
Opened	20 March 1960
Operator	London Underground

Amersham (prefix JW) is a signal cabin located within the station of the same name on London Underground's Metropolitan line as part of the Sub-Surface Railway (SSR). It is scheduled for closure under the Four Lines Modernisation (4LM) re-signalling programme.

Metropolitan Railway

The railway in this area opened on 1 September 1892 as part of the Metropolitan Railway (MR) extension from Chalfont & Latimer (then named Chalfont Road) to Aylesbury. The first signal box that opened with the line had a full-size mechanical lever frame of 28 levers. It was of similar size and design to surviving signal boxes at Chesham and Chorleywood.

In 1898, the London Extension of the Great Central Railway (GCR), the last mainline to be built into London during the Victorian era, saw services between Harrow, Rickmansworth, Amersham, Chesham, and Aylesbury shared between the MR and GCR. Whilst the GCR and MR were commercial competitors in broad terms, they maintained a close working relationship with significant interworking across passenger and goods trains being commonplace. This was reflected in 1906 with the creation of the Metropolitan & Great Central Joint Railway (MGCJR) which aimed to put into law the shared operating agreements that had sometimes led to acrimonious disputes.

In 1923, the GCR was amalgamated through the Railways Act (1921), grouping it and other geographically linked railway companies into the London & Northern Eastern Railway (LNER). Along with the London Midland & Scottish Railway, Great Western Railway and Southern Railway, these railway companies formed the ‘Big Four’ whose size was thought by the government to be an insurance policy for financial security in the years following the Great War. The MR, fiercely protective of its independence, was not included in these plans, and even though the GCR had been dissolved, the name lived on through the MGCJR. The shared running agreements via the MGCJR meant that it was not uncommon for LNER locomotives to haul MR passenger trains and MR locomotives to haul LNER goods trains.

London Transport

1933 saw the formation of the London Passenger Transport Board (LPTB), which finally integrated the MR with the other underground lines previously controlled by Underground Electric Railways of London. With this newly unified network came a desire to standardise its infrastructure and dispense with many of the operating quirks that the MR had vociferously used in the past to justify its independence. Metropolitan line services to Aylesbury were withdrawn when electrification reached Amersham in 1960, doing away with the final steam-hauled passenger services operated by London Transport.

The current signal cabin (prefix JW) opened on 20 March 1960 in conjunction with the expansion of the track layout in the area with the addition of a 3^rd through platform and a pair of reversing sidings to reflect Amersham’s newfound permanent status as the westerly terminus of Metropolitan line services. The signal cabin was equipped with an electrically interlocked Westinghouse push button desk, the commands of which were implemented via Interlocking Machine Rooms (IMR) at Amersham and Chalfont & Latimer for the Chesham Branch. The use of push-button working in de-centralised signal cabins would turn out to be the last evolutionary shift of signal cabins on London Underground. From 1965 onwards, new control rooms such as Earls Court and Coburg Street would gravitate towards amphitheatre-style control rooms and programme machines having the capability to signal entire lines as opposed to the smaller geographic confines, which gave rise to the early proliferation of signal cabins on the Underground.

1960 also marked the steady decline of the Great Central Main Line, whose goods traffic had been in terminal decline since demobilisation after WW2 and passenger traffic was increasingly diverted onto other arterial routes. While traffic levels are a shadow of the 1960s, the signal cabin has remained relatively untouched. It continues in operation today, with Chiltern Railways being the last remnants of the GCR service between London Marylebone and Aylesbury.

Four Lines Modernisation (4LM)

The introduction of the S8 and S7 stocks between 2010 and 2015 proved the catalyst for wholesale re-signalling of the SSR. Bombardier, manufacturers of the S stock trains, were also contracted to replace the 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 SSUG behind schedule and faced mounting costs and delays. London Underground terminated the contract in 2013 at an estimated cost of £900m and a completion date delay of five years.

Thales won the re-tendered contract for SSR re-signalling in 2015, renamed Four Lines Modernisation (4LM). Thales would introduce Communications Base Train Control (CBTC), a variant of SelTrac ATO. Migration to ATO on the SSR is split into 14 Signal Migration Areas (SMA), of which Amersham lies in SMA 13 concerning the railway between Moor Park, Watford, Amersham, and Chesham. Whilst the signal cabin will close with the commissioning of SMA 13, 3, and 4 aspect colour light signals will remain in place to retain interoperability with Chiltern Railways trains, which have not received modifications to be operable under CBTC.