The Panel Itself

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The panel is the means by which the signalmen give requests to the interlocking that controls the signalling equipment, and the means by which the interlocking gives indications to the signalmen.

The panel is an Integra design, heavily used by the Western Region, made by Henry Williams (Darlington).

The panel has two main parts: The lower part contains a large track layout map, formed of a mosaic of Dominos, with all the signalling control buttons and switches and the indication lamps showing the behaviour of points, signals, trains, etc. superimposed onto it. The upper part contains the train describer and, originally, the telephone concentrator. The telephone concentrator buttons were moved to the lower part of the panel when the LED berth train describer was replaced by the Vaughan Harmon VDU train describer.

Operating Fascia

The layout controlled is drawn on the panel, made up of individual square Domino tiles. There is a relatively small number of patterns of Domino, but combinations of Dominos allows almost any conceivable layout to be represented. The diagram is not to scale. In most cases tracks are by a broad black line and siding lines are sometimes shown as a hollow line. (Lines with train detection by axle counters are shown in blue on some panels, but not Swindon.) Also shown on the diagram Dominoes are the position of every signal and every set of points. All equipment has an identity which is engraved into the panel tiles (eg, points number, signal number, track circuit number, etc). Engraving the pictures and letters into the panel tiles makes for a much more hard-wearing and long-living product.

Within each track circuit is a line of lights that can show white (for when a route is set) or red (for when the track circuit is occupied), or no colour at all when neither of the previous apply.

For controlled signals, a red or green lamp will be displayed in the signal. The panel indication shows red when the signal is red (and the lamp is proved alight) and green when the signal is displaying any ‘off’ colour, eg, yellow, double yellow, green, flashing yellow, two white lights, etc. No distinction is given to the signalman regarding how ‘off’ the signal is, they just have to use their skills of knowing what colour the signals are capable of showing to work it out.

Automatic signals generally do not have indications, although those with panel replacement switches have a white and a red lamp to indicate whether the replacement switch has been used or not. In the case of these being lamp-proved the red appears in the signal in the same manner as if it was a controlled signal, but in the non-lamp-proved ones the red is offset from the signal.

Points are displayed on the panel and all shown in their ‘normal’ position (a small gap is left between the ‘normal’ and ‘reverse’ routes, with the ‘normal’ route being shown in continuous black. Each set of points is provided with an individual switch at the top of the panel for operation separate from the route-setting system. Around these switches are lamps for ‘normal’ (white, for which the switch it turned to the left), ‘reverse’ (white, for which the switch is turned to the right) and ‘locked’ (red, in the centre). The centre position of the switch leaves the points free to be set by the route-setting system, in sympathy with the routes called by the signalman. In most cases, the points will remain in the position they were last called to until they are required in the opposite position, unlike on a lever frame where the points are put back to normal after a train to keep a tidy frame. When the points are out of correspondence with their controls neither normal nor reverse lamp is lit, and also the white lamps in the track diagram immediately either side of the points flash alternately. It is normal to see this for four or five seconds while the points are in the process of moving in response to the signalman moving the individual switch or setting a route.

The method by which the signalman sets routes on the panel follows the ‘entrance-exit’ principle (NX). This means that the signalman selects where he wants a route to be set FROM and where TO, the eNtrance and the eXit, using the panel controls, and the route-setting system deals with everything in between. It is not necessary, like it would be on a mechanical lever frame, to operate all the points to the correct position before clearing the signal; this happens automatically. This switch is operated by turning it through 90° so that the required arrow points into the direction of travel. Each controlled signal will have a black rotary entrance switch next to it. Main aspect signals that lead to other main aspect signals will have a red arrow that normally lies perpendicular to the track pointing upwards. Ground position lights, and main aspects with associated draw-ahead signals underneath that read to other ground position light signals, will have a similar arrow, but in yellow and normally pointing downwards.

Each signal that forms a potential exit from a controlled signal will have an exit button next to it. Some signals can form entrances AND exits and will be fitted with both, and some only one or the other. Exits for main aspect signals, and position light routes with route indications, use red buttons. Exits for routes from independent position light signals and those associated with a main aspect but without a route indication, will use a yellow button. Exit buttons that can be used by either will be red with a yellow base.

In the process of “setting up” a route, the signalman must match the colour of the entrance switch arrow with the colour of the exit button. To set a main aspect route the red arrow is turned into the direction of travel and the next available red exit button depressed.

If the route is free to be set, the points will be commanded to move to the required position. White route lights will commence lighting up along the panel diagram, starting at the entrance and moving along to the exit. The white route lights will wait for each set of points to make detection in the required position before moving onto the next track circuit. When appropriate the signal will change colour, which may be immediate or may not, and the change in colour will be indicated to the signalman by the panel indication changing from red to green: ‘on’ to ‘off’. The signalman will set up a chain of routes in this manner to allow the train to move across the panel.

The approach of a train is indicated by the progression of red lights along the panel, as each track circuit section becomes occupied. When the train passes the signal that we just set, the signal will replace to danger automatically at the required time, and this will be indicated in the panel. As the train occupies each track circuit in the route, the white lights will be replaced with red.

As long as the route remains set, as the train vacates each track circuit in the route the red lights will be replaced with white lights. A single white lamp flashes in the first track circuit beyond the signal to indicate that the route has been ‘used’. When the route is cancelled the white lights, indicative of the route locking, will die away behind the train, allowing other routes to be set behind the train. The white lights, and hence the route locking, will never die away in front of the occupied track circuits, as this would allow points movements ahead of the moving train. A signal at the entrance of a ‘used’ route will remain at danger until the signalman cancels it and re-clears it. Some signals are provided with an ‘auto’ facility. In this case the signal will still return to danger with the passage of a train, but the route will not be considered ‘used’. The first white light will not flash and, when the advancing train goes clear of the controlling track circuits, the signal will start to step up through its aspect sequence again. This is useful where almost every train follows the same route. In the case of Swindon Panel examples of this are places like Uffington and Bourton.

In most cases automatic signals are not able to be controlled by the signalman. In the exceptional cases where they are fitted with ‘replacement switches’ these function similar to entrance switches on controlled signals. They are coloured red, with a white arrow, and are not used in association with an exit button. The signal responds to the operation of the switch alone. Routes from automatic signals are not indicated by white route lights, which gives a disjointed appearance as the panel is observed, with controlled and automatic signals interspersed throughout the area.


As we know the panel has lamps and switches on it. Part of the appeal of the design of an Integra Domino panel such as Swindon is that the diagram is formed of individual replaceable tiles, which makes modification easy.

Underneath each tile fascia is a base unit that contains the lamps (in the case of indications) or terminal pins for a switch (in the case of controls), or a mixture of both.

Each lamp terminal is individually wired to a ‘tag block’ in the back of the panel, with all lamps on a single tile having a common return wire, also back to the tag block. For controls, all contacts are individually wired back to the tag blocks, these do not make use of the common return. The wiring of base units to tag blocks follows a rigid logical numbering and colour sequence. Each tag block is given a two-letter identity. The tag blocks are arranged in columns in the back of the panel, column A being at the Uffington end, next down B, C, etc. The second letter represents the tag block within that column, counting up from A at the bottom. So as you look at the back of the panel, tag block AA will be at the bottom right, with AB above it, AC above that, etc. The next column of tag blocks along will be BA at the bottom, BB above that, etc. Not all the identities are used as some areas of the panel are more wire-hungry than others. On the tag blocks there are a large number of terminal pins. The terminal pins are identified by a number, representing the row, and a letter, representing the column.

The design of the panel Dominos will identify the geographical areas of the panel attached to each tag block, and the co-ordinate of the terminal pin to which each item (lamp, switch, etc) is wired.

Each row of the tag block is used up in order starting at 1, but not all the pins are used in each row (ie, if there are too many functions on the next tile to fit on that row, the next row will be started). All the wires from one tile will go to the same row on the same tag block. The common return always takes the last pin in the row (maximum two tiles).

There is also a colouring system for the wires. The first wire used in each tile (regardless of the terminal in the base it applies to or the tag block pin) will be red. The second blue, then green, then brown, then grey…. This helps the wires to be identified when they are soldered to the tag block. All of this wiring and soldering would have been completed in the factory before the panel was delivered to site. All the panel wires are soldered to one side of the tag block, and the other side is left bare, for the customer (Western Region) to solder their wires to for connection to the interlocking.

In this tile for SN.10, the switch is wired to terminals 12b and 12f when ‘normal’, and 12a and 12e when ‘reversed’. The red lamp indication in the signal is connected to 12h and the green lamp to 12g. The red lamp in the track circuit is connected to 12d and the white lamp to 12c. The common return is 12n. The wires will be coloured in the order they are used on the tile.

(When we come to remove the panel the cable that connects the tab block to the interlocking will be cut leaving us as much hanging cable as possible. We will open up the cut end of the cable and separate out the cores, which is what we will connect the PCB cards that will drive the simulator to (with some grouping of cores). That way we will not have to alter the tag blocks in any way in order to connect the panel to the simulator. This makes the job a lot easier and also preserves the historical integrity of the tag blocks.)

Train Describer

Swindon has had three train describers:

  • Sodeco electromechanical train describer, installed at opening, lasted until about 1985.
  • LED train describer, installed 1985, lasted until mid-1990s(ish)*
  • Vaughan VDU train describer, installed mid-1990s(ish)*, lasted until closure.

The characters displayed on the LED train describer in Bristol Panel have been recorded, in the absence of any photographic evidence from Swindon.

TD Characters

Photos of TD letters from Bristol PSB

The entering of train descriptions on the electromechanical train describer was via a rotary telephone dial. From the LED describer onwards entry was by keyboard.

  • - The LED describer was still in place in 1992 [RS].