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.