Railway Signalling in Britain: Part 3 - Slotting

'Slotting' is a technique employed in semaphore signalling which gives two signal boxes control over one signal arm and was commonly used where signal boxes were close together.

Co-located Stop/Distant Signal Slotting

A frequent requirement for slotting occurred where a distant signal controlled by one signal box was mounted on the same post but below a stop signal controlled by the signal box in the rear. The 'slot' prevented the distant arm from coming 'off' until the stop signal arm above it was also 'off'.

The picture below shows the stop signal/distant signal slot on a Western Region pattern lower quadrant signal, with both signals 'on'. The weight bar nearest the camera (with two counterweights) is for the distant, the weight bar with the single counterweight is for the 'stop' signal. With no tension in the signal wires, the weightbars are held in the position shown by the counterweights. Two 'L' cranks at the bottom of the post transfer the horizontal 'pull' on the signal wires to a vertical 'pull' via two short signal wires to the ends of the two pivoted weightbars. A push-rod is attached to the stop signal weightbar, on the counterweight side of the pivot, so that, when the stop signal wire is pulled, the counterweight rises and the push rod is pushed upwards to move the signal arm to the 'off' position. A second push-rod is provided to operate the distant signal arm but this push rod is attached not to the distant weight bar, but to a third weight bar placed in between the other two. This third weight bar is provided with a smaller weight and it also has two projecting lugs which sit on top of the other two weight bars all on the other side of the pivot from the first two weightbars. With both signals 'on' the smaller weight on the third weightbar is lifted by the other two weightbars pushing up against the lugs. The weights are arranged so that if either the stop signal or distant signal wire is pulled, the remaining lug pushing up on the third weight bar keeps the smaller weight lifted.

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View from front of signal with neither signal wire 'pulled' - both Stop and Distant signal arms 'On'.

The picture below (taken from the other side of the signal) shows the situation if both signal wires are pulled. The tension in the stop signal wire holds the counterweight lifted and the pushrod lifted upwards to move the stop signal arm to the 'off' position. The tension in the distant signal wire holds the two distant counterweights lifted. With neither of the two weightbars pushing on the lugs of the third weightbar, the smaller weight falls under gravity, pivoting the third weightbar and lifting the distant pushrod to move the distant signal arm to the 'off' position.

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View from rear of signal with both signal wires 'pulled' - both Stop and Distant signal arms 'Off'.

Stop Signal Slotted by Two Signal Boxes

There were instances where it was considered unsafe for one signal box to have total control of a stop signal and co-operation between two adjacent boxes was enforced by clearing the signal only when both boxes had operated their 'slot' levers. The actual slotting mechanism at the signal was similar to that described above for Stop/Distant Signal Slotting.

Sedgeley Junction signal box had an example. If you look at the sketch signal box diagram in the post Sedgeley Junction (again) you can see that lever 35 'slotted' Dudley East's Down Starter. This was to provide protection for a train on the Down at Coneygree. This particular instance was a bit odd because the slot stood 'off' - you only pulled the lever when you wanted to ensure that Dudley East's Down Starter remained 'on' - the lever was 'normal' most of the time, giving Dudley East sole control of the signal.

The Up and Down Goods between Deepfields and Spring Vale was another example. My Deepfields signal box diagram in the post Railway signalling: Deepfields shows the slot on signal 38 (but omits the ones on 23, 28 and 29, although I did record the complexity of the stop/distant slotting on the down: to get Spring Vales's Distant signal mounted underneath the stop arm controlled by Deepfields lever 2 to show 'off', four weight bars had to be 'off' - one pulled by Spring Vale, one each pulled by Deepfields levers 2, 3 and 4). My signal box diagram of Spring Vale in the post Railway Signalling: Spring Vale Sidings Box does show the slots on Deepfields 23, 28 and 29.

Certainly in London and North Western Railway Signal Boxes, it was common to give the signalman an indication of whether the slot operated by the other box was 'off'. This was achieved by providing 'Face Disks' behind the frame, operated by signal wire. These normally sat horizontal but, when pulled vertical by the other box, displayed 'SLOT OFF'. The excellent book 'A Pictorial Record of L.N.W.R. Signalling' by Richard D. Foster, published by Oxford Publishing Company in 1982 (SBN: 86093 147 1) has more details.

Maintenance of this mechanical complexity must have been a problem and I think there was a policy of eliminating it where possible. I'm sure it's the sort of thing to appeal to A.F. Bound (the LMS Chief Signal and Telecommunications Engineer from 1929). Increasing provision of track circuits, Block Control and electric lever locks would provide justification in some instances for the elimination of mechanical slotting. Another technique was 'Distant Indicator Working' which is briefly described in section 8 of the post Railway Signalling: Tipton (Part 2) and I believe the signalmans' regulations applying where Distant Indicators were provided probably lessened the need for traditional slotting.

Go to Part 4 - Semaphore Signal Aspects by Night.

Railway Signalling in Britain: Part 2 - Semaphore Signals

Part 1 described the evolution of fixed signals. After a long period of development, the 'modern' form of 2-aspect semaphore signalling emerged (but is now being rapidly eliminated by Network Rail).

Types of Fixed Signals

There are two principal classes of fixed signal:-

'Stop' signals - these are mandatory and an approaching driver must stop if the signal displays a 'stop' aspect.

'Distant' signals - these are warning and an approaching driver may pass the signal displaying a caution or restrictive aspect but be prepared to stop at the following 'stop' signal.

Signals comprise a vertical post (of wood, steel tube, steel lattice or occasionally reinforced concrete) on which is mounted one or more 'stop' or 'distant' signal arms each comprising a rectangular 'blade' (of wood or steel) mounted on a pivot so as to extend to the left of the signal post (viewed from an approaching train to which the signal applies) at a height to facilitate observation by the driver of an approaching train.

The two types of signal arm are distinguished by day as follows:-

'Stop' signals - Arm is square-ended. Front of arm is RED with a WHITE vertical band near the end, rear is WHITE with a BLACK vertical band near the end.

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Upper quadrant 'Stop' signal displaying 'STOP' indication (Shackerstone Outer Home).

'Distant' signals - Arm is 'V'-ended. Front of arm is YELLOW with a BLACK chevron near the end, rear is WHITE with a BLACK chevron near the end.

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Darley Dale Down (fixed) Distant Signal. The black and white painting of the post indicates an 'Independent' Distant - one not co-located with a 'Stop' Signal.

The two types of signal are distinguished by night by providing a lamp (usually paraffin), in front of which two coloured filter glasses connected to the signal arm are moved:-

'Stop' signals - Display either a RED or GREEN light.
'Distant' signals - Display either a YELLOW or GREEN light.

Aspects

Both types of signal have two valid aspects, according to the position of the signal arm (which controls the colour of the lamp shown at night):-

Arm is horizontal ('ON' Aspect): 'STOP' (if a stop signal), 'Proceed with Caution' (if a distant signal).
At night, the indications are Red light: 'STOP'(if a stop signal), Yellow light: 'Proceed with Caution' (if a distant signal).
Arm is raised or lowered by 45 degrees ('OFF' Aspect): 'PROCEED'.
At night, the indications are Green light: 'PROCEED'.

If the arm is raised by 45 degrees to indicate 'PROCEED', signal is termed 'UPPER QUADRANT' or, if lowered by 45 degrees to indicate 'PROCEED', signal is termed 'LOWER QUADRANT'.

Subsidiary signals

For special purposes (such as shunting), there are various types of 'Subsidiary signal'. These are 'Stop' signals but provided with smaller arms than 'main' signals and often mounted on the same 'doll' as a main signal.

An elderly Great Western signal. The signal arms are wooden, as is the 'doll' (vertical post) mounting the arms. The lower arm is smaller and the 'S' indicates that it authorises a shunting movement past the signal.

Diverging routes

'Stop' signals are often situated in advance of turnouts (points) where two (or more) routes diverge. Railways in Britain provide 'Route Signalling' where, at diverging lines, there is a separate signal arm for each route. Originally, these arms were placed on a single post, one above the other, with the topmost arm referring to the leftmost route. Particularly where there were more than two routes, this was judged harder for an approaching driver to correctly interpret at speed so this arrangement persisted only on slower lines, such as Goods Lines.

On main lines, individual signal arms are placed on separate, secondary signal posts carried side-by-side on a bracket from a single main signal post. Where a large number of routes are to be signalled, a gantry (or 'signal bridge') carried by two or more posts or supports can be provided. On bracket signals or gantries, the secondary posts are called 'dolls'. The leftmost 'doll' applies to leftmost route and so on.

'Stepping' relates to the relative height above ground of the arms. The highest arm is the highest speed route and so on. Two routes with the same speed limit have the corresponding signal arms 'stepped' to the same height. Note that no absolute speed is implied, requiring the driver to have a detailed route knowledge. [Foreign railways (unless under British influence) developed a system of Speed Signalling where the driver didn't know what route he was to take, only a safe speed to run at. Different signal aspects implied different absolute permitted speeds. It can be argued that this means drivers do not need such detailed route knowledge].

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Lower quadrant Two-doll bracket 'Stop' signal at Tyseley viewed from the rear. This is a standard Western Region pattern. Each 'doll' (post) carries a main arm and a subsidiary (calling-on) arm and is topped with a 'ball and spike' finial.

Positioning of Fixed Signals

Signals are provided to allow each signalman to control trains in his area. On normal double track lines, 'STOP' signals implement the 'Block System'. Each signalman became responsible for the line on which trains approach extending from the previous signalman to his own location. This was called the 'Block Section'. Only one train is allowed in each 'block section' at a time so as to avoid collisions. Each signalman communicated with the signalmen on either side using special electric telegraph instruments called 'Block Signalling Instruments'. There's a little more about 'Block Signalling Instruments' (particularly those produced by the London and North Western Railway) here.

The first 'STOP' signal at each signal box is called the 'Home Signal'. The last 'STOP' signal at each signal box is called the 'Starting Signal' (or 'Starter'). The term 'Section Signal' is increasingly used as an alternative (since the signal controls admission of trains to the Block Section for the next signal box).

Depending upon the location, a 'Starting' signal may not be provided and the 'Home' signal may also serve as the 'Section' signal. Conversely, a signal box may control additional stop signals and there may be more than one 'Home' signal on the approach to the signal box and there may be more than one 'Starting' signal.

To give an approaching driver advance warning of the aspect showing on the 'Home' and 'Starting' signals at the next signal box, a 'Distant' signal is placed in the rear of the 'Home' signal at a distance which, if the 'Distant' is 'On', will allow the driver to bring his train to a stand at the 'Home' signal bearing in mind the maximum permitted speed, the prevailing gradient and the braking characteristics of trains using the route. This distance may be a mile or more. Interlocking of the levers in the signal box is arranged so that the 'Distant' signal can only be cleared to 'Off' if the levers for all the 'Stop'signals on the signalled route have been operated.

Where signal boxes are fairly close together, the requirement to place the 'Distant' signal at a sufficient distance from the 'Home' may result in the 'Distant' signal arm being placed on the same post (and underneath) the last 'Stop' signal of the box in the rear, as shown in the picture below. In some cases, the position of the co-located 'Stop' and 'Distant' signal arms may still not offer sufficient braking distance, in which case a 'Distant' signal arm will also be fitted below a previous 'Stop' signal.

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Darley Dale's Down Home Signal - a tubular post upper quadrant signal with fixed distant for the next signal box (Church Lane) mounted below. Note the Ground Disc signal and Signal Post Telephone mounted at the bottom of the post.

Signal Sighting

It's important that a driver will be able to see a signal at the earliest possible opportunity and great care is taken in the positioning of each signal. Signal posts may be short (where an overbridge before a signal limits the driver's view), tall (where it is desired to 'lift' the arm above a visually confusing background) or cantilevered left or right to improve the driver's ability to 'sight' the signal.

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Church Lane Down Outer Home is a single doll mounted on a right-hand bracket. The 'White Diamond' sign mounted underneath the signal arm was introduced where track circuiting indicated the position of a train detained at the signal in the signal box, relieving the Fireman of the need to report to the signal box to carry out what was originally 'Rule 55'.

Mechanical Operation of Signals by Wire

Operation of the lever in the signal box (from the 'NORMAL' position to the 'REVERSE' position) pulls a stranded steel wire which is carried on a series of pulleys to the signal location where the 'pull' is used to change the signal aspect. A counterweight at the signal ensures that, if the signal wire breaks, the signal arm is returned to the 'On' aspect. The counterweight also assists in returning the arm to 'ON' if excessive friction along the signal wire route tends to prevent the signal wire moving back to the original position when the lever in the signal box is replaced to 'NORMAL'.

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Single signal wire carried on a 2-way alloy signal pulley. The pulley is mounted on a vertical post driven into the ground (in this case a length of U-channel point rodding).

Where signal wires need to change direction (for instance, to cross to the other side of running lines), the change of direction is made on horizontal pulley wheels. A length of chain is interposed into the signal wire in the vicinity of the pulley to provide sufficient flexibility to accommodate 90 degree changes of direction. Depending upon the complexity of the layout, one or more pulley wheels may be fitted to a cast iron carrying frame which is fixed to a wood or concrete 'bed' set into the ground. These pulleys are normally referred to as 'Chain Wheels'.

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Single chain wheel providing around 90 degree change of direction to the signal wire. The frame of the chain wheel is bolted to a concrete 'bed'.

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A more complex run of signal wires (just to the left of the temporary fencing) at Tyseley. In this case, the pulleys are carried on flat steel spikes driven into the ground and a 4-way signal pulley is fitted back-to-back with a 3-way pulley.

There's an article on semaphore signalling on the London and North Western Railway here.

Go to Part 3 - Slotting.