THE SIGNAL BOX

SIGNALS

BRITISH RAILWAY SIGNALS

LMS SPEED SIGNALLING

LMS Speed Signal at Keighley Museum
Photograph by
Nick Wellington

To the overseas student of railway signalling the concept of speed signalling is not difficult to grasp, but in the UK it is often regarded as something totally alien. Modern British Colour Light signals are incredibly simple in principle but always give an indication of what route a train is to take at a junction. Speed signalling, in contrast, does nothing of the sort - it simply gives information of the speed that a junction may be taken at. Maybe the railways of Britain are just too complex both in layout and by traffic pattern for speed signalling to have ever been practical.

One sole installation of speed signalling was made in Yorkshire by the LMS in 1932. This was installed on a three mile stretch of quadruple line between Heaton Lodge Junction and Thornhill LNW Junction which included a junction and station at Mirfield. The scheme was for the signals alone, and whilst a little sensible thinning-out of signal boxes occurred, the control remained from mechanical frames and the Absolute Block system continued in use.

All signals in the Mirfield scheme were of the searchlight type, using multiple numbers of heads to achieve the desired combination. A "marker" light was fitted lower down the post; this provided a secondary red light in case of failure of the bulb(s) in the main head.

Three speed terms were used in connection with this system.

  1. High Speed - Normal line speed
  2. Medium Speed - Speed through junctions (including those between running lines)
  3. Low Speed - speed through connections into sidings

Note that the terms Preliminary Caution and Advance Caution used below are unofficial - in the official documentation these indications were loosely referred to as Attention indications.

Although speed signalling was never adopted elsewhere in the UK, this small enclave survived until May 1970, when it was swallowed up by a larger power signalling scheme.



Stop signal

Speed signalling - stop signal

A straightforward stop signal, without any junctions ahead, was capable of showing up to five indications - giving very similar indications to the multiple-aspect signals used today.

Indication Meaning   Indication at next signal
R-R Stop   -
Y Caution High speed Stop
Y-Y Preliminary Caution High speed Caution
Y-G Advance Caution High speed Preliminary Caution
G Clear High speed Clear, or Advance Caution

When the signalling was first brought into use, the marker light was illuminated at all times except when a single green aspect (or a subsidiary aspect - described below) was displayed. By 1937, however, the arrangement had been simplified to that described here.

Junction signal

Speed signalling - junction signal

At a running junction, a third main head is added to the signal. The top indication applies for "main line" running, whilst the bottom of the three applies to the lower-speed route at the junction, whether it be to left or right. The middle head provides the additional indications for both routes.

Indication Meaning Route Indication at next signal
R-R-R Stop   -
Y-R-R Caution High Speed Stop
Y-Y-R-R Preliminary Caution High Speed Caution
Y-G-R-R Advance Caution High Speed Preliminary Caution
G-R-R Clear High Speed Clear, or Advance Caution
R-Y-R Caution Medium Speed Stop
R-Y-Y-R Preliminary Caution Medium Speed Caution
R-G-R Clear Medium Speed Clear, or Preliminary Caution

These signals were only used for routing between parallel running lines. At physical junctions, multiple-head bracket signals were used - see below.

The system was simplified in 1959 by discontinuing the Advance Caution indication, and two successive Preliminary Caution indications would be displayed if inadequate braking distance existed. It is interesting to note that the Advance Caution indication provided a feature not since catered for by British signalling*. As line speeds continue to increase, demand for an "Outer, Outer Distant" continues to rise, but the present signalling system cannot cope with such a need other than by placing a second double yellow in rear of the one that has insufficient braking distance. This method is flawed by giving the driver the illusion that he must stop sooner than intended - even, perhaps, in a distance that the driver knows is not achievable - leading to excessively hard braking and causing unnecessary worry to drivers.

* - a fifth indication (flashing green) has been used experimentally at locations on the East Coast Main Line for 140 m.p.h. running.

Multi-head Junction signal

Spped signalling - multi-head junction signal

At physical junctions, separate signal heads were provided so that the driver had a positive indication of route. This was really a sign that the LMS was not prepared to commit themselves 100% to the principles of speed signalling, fearing problems with wrongly routed traffic.

This example, which represents the Down Slow Starting signal at Mirfield No3, has three dolls:

Left-hand doll Centre doll Right-hand doll
To the Cleckheaton branch To the Down Slow To the Down Fast
No marker light was provided because the signal led to a conventionally-signalled area. A yellow aspect acted as the distant signal for Mirfield No4. A marker light is provided. Only three speed indications are given, because there is no need for a preliminary caution aspect as signal spacing is adequate. A marker light is provided. Only three speed indications are given, because there is no need for a preliminary caution aspect as signal spacing is adequate.

Splitting Distant signal

 

Where it was deemed necessary to provide a splitting distant signal to indicate the route cleared at the next signal ahead, things begin to look complicated. Two standard principles of LMS signalling of the era were combined. This was not really a speed signal at all!

The signal shown here represents the Up Advanced Starting signals at Mirfield No1, which led onto a semaphore-signalled area at Heaton Lodge Junction. At Heaton Lodge Junction, the four-track main line continued towards Cooper Bridge, with running junctions between these lines. Four tracks also diverged left towards Huddersfield.

At this time, the LMS used a double-yellow aspect to indicate that a route is clear throughout via a lesser speed route at a junction, whilst green would indicate clear for the faster route. A signal of this type was involved in the Bourne End accident of 1945, and whilst the signalling was not directly implicated the practice of using double-yellow aspects for this purpose was subsequently discontinued.

The LMS also used a three-doll system, which displayed a green light alongside a yellow light. The central lamp was the green, and the yellow lights were illuminated according to the routing circumstances. The combination identified the route cleared - if the green was to the right of a yellow, the right-hand route was cleared, and vice-versa.

Here, on the approaches to Heaton Lodge Junction (and similarly at Thornhill LNW Junction) the two principles were combined so as to show both the direction to be taken at the junction ahead and whether the train was running via a slow or fast route. The principle can be baffling, but is easier to understand if you keep in mind that the outer lamps are there solely to indicate whether the train is routed to the left or right.

The indications that could be displayed by this signal were as follows:

Splitting distant signal
Indication Meaning Route Indication at next signal
  R   Stop   -
  Y   Caution   Stop

Y
G   Clear Along Up Fast
or
Along Up Slow
to
Cooper Bridge
(main route)
Clear

Y
Y

Y
  Clear Up Fast to Up Slow
or
Up Slow to Up Fast
to
Cooper Bridge
(main route via running junction)
Clear
Y

Y

Y
Clear

Up Fast to Up Huddersfield South
or
Up Fast to Up Huddersfield North
or
Up Slow to Up Huddersfield South
or
Up Slow to Up Huddersfield North
(branch routes)

Clear

The double-yellow indications were removed by 1960, and their function replaced by the single yellow aspect. This just left the yellow/green combination in place, which survived until the renewal of the signalling in 1970.

Call-on signal

Spped signal - call-on

On Permissively-signalled lines, an indication that a train may proceed with the section ahead occupied was given by a miniature yellow light beneath the marker light. The marker light was extinguished when the miniature yellow aspect was lit.

Where a signal read to a Goods Loop signalled on the "No Block" system, a Medium Speed junction signal would be provided, although it would be non-operative. Instead, the Call-on aspect would operate for all trains to that line.

Siding signal

Speed signal - siding

Facing connections into sidings (low speed) were catered for by a miniature green aspect beneath the marker light. The marker light was extinguished when the miniature green aspect was lit.

At most locations, both Siding and Call-on signals would be found below the main aspects. The photograph at the beginning of this article shows a signal of this type, now preserved in the railway museum at Oxenhope on the Keighley & Worth Valley Railway.

Warning arrangement

No separate indication was given to trains accepted under the Warning Arrangement (Regulation 5). They would be brought to a stand at the signal before the signal was cleared, if the line was only clear as far as the next signal with the clearing point fouled.


Photographs of the signalling · Track Layout Plans

With thanks to Clive D W Feather, Dave Harrison, Mike Hodgson, Simon Lowe and Nick Wellington for their assistance during the preparation of this article.