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Section 6: Junction Signals and Route Indicators

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A three-aspect signal (number UC4B) had been installed in 1935 at Hackney Downs with two directing distant indications, stepped in height (see [6.35]). Later in the same year, it was converted to a four-aspect signal and the method of displaying advance routeing information was altered to a form somewhat reminiscent of the suggestion in the IRSE Three-Position Signalling Committee's report of 1924 that certain colour light junction distant signals should be fitted with indicators that display either "L" or "R", for the left-hand or right-hand route. The directing distant indications were removed and replaced with special route indicator lamps. When the junction signal and the distant signal were both cleared, one of the special indicators attached to the latter would show an indication "F" or "S", referring to the Up Fast or Up Suburban line, respectively. The indicators were appropriately positioned on either side of the signal post and stepped in height [6.36 & 6.37]. They remained in use until 2001. This form of advance route indicator was not replicated elsewhere.

[6.36] Route Indicator Lamp indicating junction signal ahead cleared for route to Up Fast line (e.g. 'clear' aspect).
Area: Hackney Downs, LNER   Usage: Low   Status: Historical
[6.37] Route Indicator Lamp indicating junction signal ahead cleared for route to Up Suburban line (e.g. 'preliminary caution' aspect).
Area: Hackney Downs, LNER   Usage: Low   Status: Historical

In what was to become the standard form of colour light splitting distant signal, multiple signal heads are positioned side by side and stepped in height according to the relative speeds of the routes. This is the direct colour light equivalent of a splitting distant signal in semaphore form (see [6.5]). If required to also act as a stop signal, one of the heads in the splitting distant signal will be capable of displaying a red 'danger' aspect. When the junction signal was at 'danger', the splitting distant signal (provided it was not itself at 'danger') would display a single yellow aspect in each of its heads [6.38]. When the junction signal was cleared, the appropriate head would step up to a higher aspect (double yellow or green) [6.39]. It was exceptional for this form of splitting distant signal to have as many as three heads.

[6.38] Colour Light Splitting Distant Signal showing single yellow in each head (e.g. three heads).
Area: All Areas   Usage: Medium   Status: Obsolescent
[6.39] Colour Light Splitting Distant Signal (e.g. junction signal ahead cleared for a (lower speed) left-hand route; signal at 'preliminary caution'). Click Here for Photo
Area: All Areas   Usage: Medium   Status: Current

On the Cheshire Lines Committee, a variant form of directing miniature colour light shunting signal (see [6.28 & 6.29]) was used where associated with a main signal (see also [3.71]). When cleared for the left-hand route, a green light was displayed above a letter "S" [6.40] and, when cleared for the right-hand route, a green light was displayed below the letter "S" [6.41]. No aspect was displayed when the shunting signal was 'on'.

[6.40] Directing Miniature Colour Light Shunting Signal associated with a main signal, 'off' for left-hand route.
Area: CLC   Usage: Low   Status: Historical
[6.41] Directing Miniature Colour Light Shunting Signal associated with a main signal, 'off' for right-hand route.
Area: CLC   Usage: Low   Status: Historical

The LNER's experimental junction type route indicators at Thirsk (see [6.31 & 6.32]) were judged a success and became a standard feature, although in a modified form. The vertical indicator for the straight route (see [6.31]) was deemed unnecessary and was dispensed with. Future junction indicator arms comprised a row of individual white lights, a throwback to A.E. Tattersall's earlier experiments with position light type main signals (see [2.78 - 2.80]).

On any individual colour light signal, a maximum of three junction indicator arms can be placed to the left-hand side and, likewise, a maximum of three to the right-hand side. The six permitted junction indicator positions are referred to by number; positions 1, 2 and 3 are for routes to the left of the straight route and positions 4, 5 and 6 are for routes to the right [6.42]. If only one diverging route exists to the left or right of the straight route, it will be indicated by a position 1 or position 4 junction indicator arm, as appropriate; these positions are inclined at an angle of 45° from the vertical. Additional diverging routes can be catered for by adding further indicator arms (up to a maximum of three, as already stated) at progressively lower heights and increasing angles of inclination. Each additional arm is inclined at an angle of 45° greater than the one immediately above it. Where a signal is provided with more than one junction indicator arm on the same side (e.g. positions 1 and 2), the indicator at the lowest height applies to the route that diverges furthest from the straight route. Junction indicator arms are commonly referred to as 'feathers'.

The earliest position light type junction indicators were composed of three white lights in each arm [6.43]. The Southern Railway began installing this type with the 1936 resignalling between Waterloo and Hampton Court Junction. The LNER installed some of the three-lamp type in 1938 on the Chingford Branch resignalling scheme.

[6.42] Junction Indicator Positions.
Area: All Areas   Usage: High   Status: Current
[6.43] Three-Lamp Junction Indicator (e.g. Position 1). Click Here for Photo
Area: Predominately Southern Railway   Usage: High   Status: Historical

A main signal with a junction indicator may also have a subsidiary signal (see Section 4) that can apply along the route(s) for which the junction indicator is provided. In some such cases, the junction indicator will be displayed when either the main signal or the subsidiary signal is cleared [6.44]. In other cases, where the junction indicator is only displayed with the main aspects, the subsidiary signal may have its own associated miniature alphanumeric route indicator.

A junction indication and an alphanumeric route indication can be displayed simultaneously at the same main signal [6.45]. This arrangement occasionally has to be resorted to in situations where the number of routes diverging on the same side of the straight route exceeds the maximum three divergences that a junction indicator alone can refer to. The same arrangement may be employed to avoid having to provide a junction indicator with both positions 1 and 6 or positions 3 and 4; these combinations of opposite junction indicator positions are considered undesirable because there is a risk that the indications may be misread.

[6.44] Junction Indicator used with a Subsidiary Signal (e.g. three-lamp junction indicator with 'calling-on' disc).
Area: All Areas   Usage: High   Status: Obsolescent
[6.45] Junction Indication and Alphanumeric Route Indication displayed together (e.g. three-lamp junction indicator with theatre type route indicator). Click Here for Photo
Area: All Areas   Usage: Low   Status: Current

On the LMS and the LNER, the preference was for an alternative form of position light junction indicator with five white lights in each arm [6.46]. Following nationalisation, the five-lamp type was standard on all regions of B.R. apart from the Southern Region, which stuck with the three-lamp variety (see [6.43]). Discussions took place in the early 1960s with a view to selecting one form of junction indicator as standard for use across all the B.R. regions. While the Motive Power Committee considered that the five-lamp type was the more effective option, the S&T Committee's preference was for the cheaper and simpler three-lamp indicator. 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.

[6.46] Five-Lamp Junction Indicator (e.g. Position 2). Click Here for Photo
Area: All Areas   Usage: High   Status: Current