The previous article described the various arrangements used for signalling junctions.
All of these arrangements have disadvantages. Approach control doesn't give the driver warning of the divergence, and runs the risk that drivers will expect red signals to clear as a matter of course. Flashing aspects don't provide a clear reminder when the diverging route is not set. Finally, although splitting distants don't have those disadvantages they do require a complex signal head which is ungainly when more than two routes are involved.
In 1998 Railtrack published a set of proposals for a new system for signalling junctions. This was based on the following principles:
Note: in most of the diagrams that follow the aspects have been drawn with the bottom three lamps in the same place. This has been done to conserve space; the real signals continue to have one head for each colour.
These criteria are met through a new device: the junction warning
indicator. This is a box mounted under the signal that (using fibre-optic
technology) shows an arrow:
The junction warning indicator, or JWI, is mounted under the signal before the junction and is lit whenever a route is set past the junction signal (that is, if the signal carrying the JWI is double yellow or green). The direction of the arrow matches that of the junction indicator, with an arrow pointing up used when there is no junction indicator (for the straight ahead route).
In four aspect areas the signal before that carrying the JWI has a preliminary
junction warning indicator, or PJWI. This is identical to the JWI but is
This is also mounted under the signal and is lit only when relevant
(normally this will only be when the signal is green). It can be omitted if the
layout is such that the JWI on the following signal provides sufficient warning
(this could be the case, for example, if there is a speed restriction between
the two signals). The following diagrams show the operation of the system:
To ensure that the driver is aware that a given divergence requires a reduction in speed, a warning board (a yellow-edged triangle with an arrow above) is placed at the signal before the point at which braking would need to start, and the AWS of that signal and all subsequent ones up to the junction will generate a warning even if the signal is green.
Where there is more than one junction in succession, the JWIs and PJWIs are
simply mounted under the appropriate signals and operate independently of one
another. If a signal carries both a JWI and a PJWI, the latter is mounted below
the former and only lights for those routes where there is a second junction.In
a particularly complex case, a signal might carry all three kinds of indicator -
junction indicator, a JWI, and a PJWI - all lit at once:
For example, with junctions at consecutive signals (only the signals
applying to an approaching train are shown):
The following diagrams show how the new aspects would be used in a
real-world example. The location is the Great Western main line around Airport
Junction, but with bidirectional running added to the main lines (there is some
confusion as to whether this is present or not). A train approaching on the Down
Main has no less than 7 choices of route; to Heathrow:
remaining on the Down Main:
wrong direction on the Up Main, crossing after the junction:
or crossing at Hayes & Harlington:
across to the Down Relief:
wrong direction running on the Up Relief:
and finally into the bay platform (note the yellow aspects):
The new aspects also have better behaviour in the event of failure. The JWI on a signal must be proved alight before it can change to double yellow, and the PJWI before it can change to green (it is already the case that a junction indicator must be proved alight before a signal can clear). Both these tests are easily provided in the signalling and require no special arrangements (unlike the case, for example, where the flasher fails on a flashing yellow signal). To the driver these failure modes look as if the signalman has simply delayed clearing the junction signal.
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