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In 1912, the London & North Western Railway (LNWR) opened a new double-track line from Camden to Watford Junction to provide additional capacity for suburban passenger traffic. The 'New Line', as it was (and still is) known, ran mainly alongside the existing West Coast Main Line and was electrified with the fourth rail DC system at a voltage of 630 V. In 1915, the London Electric Railway's Bakerloo Line was extended to Watford Junction via the LNWR's new line, the LER's tube trains sharing the tracks with the LNWR's own electric trains. As a result, the line carried a very intensive service. Ownership of the line passed to the London Midland & Scottish Railway (LMS) in 1923.
The Signal Engineer of the LMS, A F Bound, devised a signalling system comprising colour light signals and continuous track circuiting, which was brought into use over the Watford New Line in stages during 1932/1933. Most of the new signals were either automatic or semi-automatic signals capable of working automatically when the relevant lever in the signal box was left in the reverse position. Signals along most of the plain line sections were alternately stop signals and repeater signals (figure 1). Most stop signals could display only a red or a green aspect, although a few could also display yellow or double yellow aspects where the next signal was also a stop signal. Each stop signal was fitted with a trainstop, which was raised when the signal displayed a 'danger' aspect. Repeater signals could display a red, yellow or green aspect but were not provided with trainstops. Each signal's aspect was automatically replaced to red on the passage of a train.
Nearly every signal was fitted with a red marker light, located about four feet below the main signal head. Any signal that read towards a semaphore signal or a London Underground signal was not provided with a marker light. The marker lights fitted to stop signals were mounted vertically below the main aspect, whereas those fitted to repeater signals were offset to the left by about ten inches. The marker light remained illuminated except when the main signal was displaying a green aspect or when a subsidiary aspect was displayed.
|Fig. 1: Stop Signals and Repeater Signals.|
After a few years in service, the marker lights were altered to be illuminated only while the corresponding main signal was displaying a 'danger' aspect (and no subsidiary aspect was displayed).
A peculiar feature of the signalling was the automatic calling-on facility. Although this was primarily intended to keep trains moving in the event of a track circuit failure, it could result in more than one train occupying a signal section at the same time. About a minute after a train came to a stand at a stop signal which was displaying 'danger' and working automatically then, providing the first track circuit beyond the signal was clear, the trainstop was lowered and the red marker light was automatically extinguished and replaced with a small yellow calling-on aspect (figure 2). The train could then proceed cautiously into the section ahead, its driver being prepared to stop short of any obstruction that might be encountered. It was under this circumstance that a train could arrive at a repeater signal displaying a red aspect. As mentioned above, the marker lights on repeater signals were offset to the left. A red aspect together with an offset marker light was a 'stop and proceed' indication which, after waiting for one minute, a train was permitted to pass and proceed with caution towards the next signal.
|Fig. 2: Automatic Calling-on Facility.|
A telephone on an omnibus circuit was provided at each semi-automatic signal (denoted by a "T" plate on the signal post), which the driver could use to communicate with the signalman if detained at the signal. Telephones were not provided at automatic signals. If a train was brought to a stand at an automatic signal (identified by an "A" plate on the signal post) and the marker light failed to change to a small yellow aspect, then the train was permitted to pass the signal at 'danger' after waiting for three minutes, and proceed cautiously towards the next signal.
In the event of the current in the conductor rails being cut off from any section of the line, the signals protecting entry into the isolated section at each end were placed and maintained at 'danger', to prevent a train from bridging the gap. In addition to being held at 'danger', an indicator fitted on the post of each protecting signal became illuminated to display the words "track dead" (figure 3). Trains were not permitted to pass an illuminated "track dead" indicator under any circumstances.
|Fig. 3: Section Gap protected by Stop Signal.|
Where the first signal on the approach side of the section gap was a repeater signal, then that signal was also equipped to act as a protecting signal, in addition to the preceding stop signal (figure 4). A repeater signal in this situation required to be fitted with two marker lights, one offset to the left for normal working and one vertically below the main signal head for section gap protection.
|Fig. 4: Section Gap protected by Stop and Repeater Signals.|
The Watford New Line was resignalled with conventional colour light signals in December 1988.
A broadly similar signalling scheme to that on the Watford New Line was brought into use by the LMS on the Local lines between Barking and Upminster, in September 1932. These lines were electrified and were also used by London Underground trains. The principal difference with the signalling was the absence of any repeater signals, each stop signal being capable of displaying a yellow aspect to function as a repeater for the next stop signal ahead. An automatic calling-on facility was provided. After the line was transferred to London Underground ownership in the 1960s, it was resignalled to London Underground standards.