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Section 6: Junction Signals and Route Indicators

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On the LMS, a miniature green signal (see [3.85]) could be provided to the left-hand or right-hand side of a main signal. In some cases, two miniature green signals could be provided on the same side, for route indicating purposes. Where this occurred, each miniature green signal was provided with a miniature red aspect to be illuminated when the green light was extinguished [6.51], for the purpose of distinguishing which signal had cleared.

[6.51] Miniature Green Signals (e.g. 'off' for route to the line furthest to the left of the main line).
Area: LMS   Usage: Low   Status: Historical

Discussions took place in the early 1960s with a view to selecting one form of position light junction indicator as standard for use across all the British Railways regions. While the Motive Power Committee considered that the five-lamp type (see [6.50]) was the more effective option, the S&T Committee's preference was for the cheaper and simpler three-lamp indicator (see [6.46]). The five-lamp junction indicator was finally accepted as standard, although the Southern Region continued to install the three-lamp type until around 1967, the last example of which was abolished in 2016.

In later examples of colour light splitting distant signals, the single yellow aspects in each signal head were exhibited alongside each other at the same height [6.52]. It was considered to be unnecessary for the aspects to be at different heights when the junction signal ahead was at 'danger'.

[6.52] Colour Light Splitting Distant Signal showing single yellow in each head. Click Here for Photo
Area: All Areas   Usage: Low   Status: Obsolescent

The arrival of fibre-optic signal technology in the early 1980s led to the introduction of a new form of route indicator to replace both the theatre type (see [6.24]) and stencil type (see [6.19]) indicators. There are two sizes, known as 'standard' and 'miniature' [6.53]. Standard indicators are usually associated with main signals, while miniature indicators are usually associated with shunting or subsidiary signals. Route indicators that display characters are now known generically as 'alphanumeric' route indicators, regardless of the technology used. Fibre-optic technology in alphanumeric route indicators subsequently gave way to LED lighting.

In March 1985, signal PN249 at Lancaster was provided with an experimental type of fibre-optic route indicator, which was subsequently fitted to several signals in the Crewe resignalling scheme later in the same year. A fibre-optic indicator, appropriately offset to the left or right of the main aspect, could display a rectangular bar of white light at the angle required to represent a junction indication [6.54]. The same indicator could display an alphanumeric route indication if required for other routes. Further route indicators of this type were provided on signals P423 and P468 at Peterborough in 1987; these were removed in 1989.

[6.53] Alphanumeric Route Indicator ( (a) - Standard Indicator; (b) - Miniature Indicator ).
Area: All Areas   Usage: High   Status: Current
[6.54] Junction Indicator displaying a bar of light (e.g. Position 1).
Area: All Areas   Usage: High   Status: Current

Colour light splitting distant signals were re-introduced as a current feature of British signalling practice in 1992, having been discontinued upon the introduction of flashing yellow aspects (see [2.130 & 2.131]). Concurrent with the re-introduction of splitting distant signals, a new feature appeared, namely the 'preliminary route indicator' (PRI). This was to be installed some distance in rear of a colour light splitting distant signal in a four-aspect signalled area, where serious operational delays would occur if a train was to be wrongly routed at the junction. The PRI is blank when the splitting distant signal ahead is showing either a 'danger' aspect, or a 'caution' aspect in each head. When the splitting distant signal shows an aspect better than single yellow in one of its heads (and the junction signal is therefore displaying a 'proceed' aspect), the PRI will exhibit a white arrow indication. If the junction signal is cleared for the highest speed route (without a junction indicator being displayed), the PRI will display a vertical arrow [6.55]. When the junction signal shows a 'proceed' aspect together with a junction indication, the PRI will display an arrow pointing in the same direction as that junction indicator [6.56].

[6.55] Preliminary Route Indicator (junction signal ahead cleared for straight route). Click Here for Photo
Area: All Areas   Usage: Medium   Status: Current
[6.56] Preliminary Route Indicator (e.g. junction signal ahead cleared with Position 3 junction indicator). Click Here for Photo
Area: All Areas   Usage: Medium   Status: Current

A PRI had been provided in 1991 on a trial basis between Southall and Hayes (Western Region). Drivers were instructed to disregard its indications. This PRI was not permanently brought into use until 1997 when Airport Junction (for the new line to Heathrow Airport) was commissioned. By that time, the PRI was regarded as obsolescent and although it seemed likely that no more would ever be installed, this turned out not to be the case.

In October 1994, an experimental alteration was made to the junction indicators on two signals approaching Exeter St. David's station (signal E35 on the Up Main and signal E56 on the Down Main). As an aid to determining the applicable direction of the junction indication, the 'pivot' lights on both indicators were fitted with a blue lens; thus, an illuminated junction indication appeared as one blue light and four white lights [6.57]. The blue pivot lights were removed and the standard white lights reinstated in October 1995.

[6.57] Experimental Blue Pivot Light in five-lamp junction indicator (e.g. Position 6).
Area: Exeter St. David's   Usage: Low   Status: Historical

A number of changes were made in 1996 to the principles concerning colour light splitting distant signals. In future, when a splitting distant signal reads up to a junction signal at 'danger', just one single yellow aspect will be displayed (see [2.105]), rather than one in each head (see [6.52]).

The 1996 principles changes also saw the introduction of the 'outer splitting distant signal'. In four-aspect signalled areas, the signal in rear of a splitting distant may be equipped as an outer splitting distant signal where routeing information needs to be given earlier than at the 'inner' splitting distant. Where an outer splitting distant is provided, the signal preceding the junction signal becomes known as the inner splitting distant signal. The outer splitting distant will display a double yellow aspect when the inner splitting distant is displaying single yellow and the junction signal is at red (i.e. a normal four-aspect sequence). When the junction signal is cleared, the outer splitting distant signal will show a green aspect in one head and double yellow in the other [6.58]. The green aspect is on the side corresponding to the direction of route that is set for the train and the aspects are stepped in height according to the relative speeds. While the introduction of the outer splitting distant signal had been intended to make the preliminary route indicator (see [6.55 & 6.56]) redundant, these did see further use, albeit under different circumstances. No example of this form of outer splitting distant signal was ever installed.

The principles concerning outer splitting distant signals were revised in 1999. When one signal head cleared to green, the other head would display a single yellow aspect in place of the double yellow aspect previously specified. The aspects were stepped in height according to the relative speeds at the junction. Thus, the aspect displayed by the outer splitting distant signal would be identical to that displayed by the inner splitting distant, if it is also showing green in one of its heads. No example of this form of outer splitting distant signal was ever installed.

In 2002, the principles for outer splitting distant signals changed again. The new principles saw a return to 1996 practice whereby one head would display a double yellow aspect while the other was showing green; however, the green aspect is positioned higher or lower than both yellow lights in the other head [6.59], instead of being at equal height with one of them. The first outer splitting distant signal was introduced in September 2015, on the approach to Bicester South Junction.

[6.58] Outer Splitting Distant Signal (e.g. junction signal ahead cleared for a (higher speed) right-hand route).
Area: All Areas   Usage: Low   Status: Historical
[6.59] Outer Splitting Distant Signal (e.g. junction signal ahead cleared for a (higher speed) left-hand route).
Area: All Areas   Usage: Low   Status: Current

Preliminary route indicators (see [6.55 & 6.56]) made a return in 2002 when two were provided at Saltwood Junction. Unlike the earlier installation at Airport Junction, no splitting distant signals were involved in the junction signalling arrangement. The PRIs supplement a compliant junction signalling arrangement, conveying advance routeing information for the sole purpose of preventing mis-routeing of trains. A number of similar installations have since been made and their provision is not necessarily confined to four-aspect signalling territory. In four-aspect territory, two PRIs in succession are normally provided on the approach to the junction. A PRI will only display an indication when the relevant junction signal is displaying a 'proceed' aspect and, where applicable, any signal situated between the PRI and the junction signal is also displaying a 'proceed' aspect.

To comply with the existing standard, the first junction indicators to use LED clusters instead of filament lamps had perpetuated the traditional 'position light' appearance (see [6.50]). A new form of LED junction indicator, which displays a solid bar of white light in each arm (see [6.54]) instead of five individual lights, went on trial in August 2010 at Pelaw (signal number T49) and was subsequently accepted for general use.