Now, a reliable method of train detection is crucial to the integrity of any railway signalling system, and with this in mind a great deal of thought was given to the matter. The problem with any outdoor-railway is that no matter what size or scale it is, the ability to maintain a continuous electrical contact between the railhead and the wheel-faces of a train’s vehicles cannot always be guaranteed, particularly after long a period of disuse has occurred. To that end, any thought of electrical track circuiting for the ‘company’s’ new signalling system was quickly discarded.

The next system to be considered by the ‘company’s management’ was something that is known as a ‘Latch-In/Latch-Out’ system – This is where the leading wheels of the train strike some kind of rail-mounted treadle as they both enter and then leave the section of line concerned. But, as this system only registers the leading wheels of each train, it can lead to the premature clearance of the signal protecting the section, as the rear portion of the train would still be occupying it. Although not suitable for the use with the signalling system in question here, one way round this is to use a method of signalling known as ‘Double Block Working’. This is where the train must be in the process of leaving the section in front of the section concerned before the protecting signal can show a ‘Proceed’ aspect for the next train – What was needed now as far as this ‘company’ was concerned though, was a system that would immediately detect when the complete train had passed over the Train Detector at the exit of the section in question.

For its new signalling system, the ‘company’ ordered a new Centralised Traffic Control (C.T.C.) machine, and it specified to the ‘supplier’ that it had to be able to automatically control the whole system from a location cabinet that had previously been provided with a mains electricity supply at the mine end of the railway.

Using electrical circuits that had been previously developed by the ‘company’s signalling department’, the C.T.C. machine was built by the Millfield Workshops and installed in the location cabinet as specified. The entire installation consists of ten six-pole changeover relays on a rack, together with their associated timing circuits, and a diode matrix. Two transformers each with their associated protection circuitry, provide both 12v for the signal indications and 35v D.C for powering the relays respectively.

Automatic Centralised Traffic Control Machine at Coolamusta Mine (Front on the left, and rear on the right) - The whole assembly is hinged to allow access to the rear of the machine....


There are two signalling sections in use on the Consolidated Shales Heavy-Haul Railway, one for empty trains using the Southbound Main line, and one for loaded ones using the Northbound Main Line. The entrances to each section are each provided with a two-aspect colour light stop signal, whilst the exits are each provided with a ‘Proceed On Site Authority’ (POSA) Board for entry into the respective yard areas.

Left: 'Proceed On Sight Authority' (POSA) Yard Entrance Board - Right: Yard Exit Signal



The basic electrical circuits for each track section within the C.T.C. machine, each incorporates three relays. These relays are known consecutively as the ‘Signal’ Relay or ‘GR’ (which controls the signal protecting the entrance into the section), and then the ‘Priming’ and ‘Reset’ Relays, both of which tell the system when the train has been deemed to have passed completely clear of the detector at exit of the section. Each of these relays has what is known as a ‘Slugging’ Circuit associated with it, to provide a controlled timing of the de-energisation of the relay, together with the associated changing over of its contacts during the operation cycle. Basically, each ‘Slugging’ Circuit consists of an electrolytic capacitor of a pre-determined value that is placed in parallel across the coil of its associated relay. An independent 82ohm 0.5W resistance is also placed in series with this combination. This last component is essential to prevent contact burnout within the rest of the associated circuitry, as the ‘Slugging’ Capacitor will present a momentary short circuit when in a discharged state. In the case of the ‘GR’ and ‘Reset’ Relays, the release time has been set to half-a-second to allow for the changeover of contacts during the operation cycle, whilst in the case of the ‘Priming’ Relays, the release time has been set so that they will stay in a fully energised state for a period of five seconds after disconnection. This is the period of time that has been found to be the minimum required to ensure that the last wheel-set of the train has passed over the detector at the exit of the section concerned!

When a section of line is clear, the ‘GR’ controlling its protecting signal is energised and self-held (or latched) over one of its own contacts in what is known as a ‘Stick Circuit’. This enables a ‘Proceed’ aspect to be given by its associated signal, ready for the next train to pass through the section. When a train is detected at the entrance of the section, the detector there will break the ‘Stick Circuit’ on the ‘GR’, and the relay will de-energise. This will cause the protecting signal at the entrance of the section to display a ‘Danger’ aspect, so as to prevent any further trains from entering the section – When the train is next detected at the exit of the section, the associated ‘Priming’ Relay is first energised by the detector there. This in turn will energise its associated ‘Reset’ relay. Then, five seconds after the passage of the last vehicle of the train, the ‘Priming’ Relay will de-energise, momentarily leaving the ‘Reset’ Relay in an energised condition. The resulting momentary combination of the ‘Priming’ Relay being de-energised and the ‘Reset’ Relay being energised will in turn re-energise the ‘GR’ for a sufficient amount of time for it to be able to ‘Latch’ over itself again, and so enabling a ‘Proceed’ aspect on the protecting signal to be shown, ready for the next train. The final part of the operating cycle now involves the ‘Reset’ Relay, and all that happens here is that when its associated ‘Slugging’ Capacitor has been drained, the relay will de-energise ready for operation by the next train.

Consideration has also been given in the circuit design for any unauthorised wrong direction movements. Therefore, should a train be detected at the exit of the section before one has been detected at the entrance, the system will automatically break the ‘Stick’ Circuit on the ‘GR’, and hence de-energise the relay so that the protecting signal will be made to show a ‘Danger’ aspect!


On the Consolidated Shales Heavy-Haul Railway, the Signals, ‘POSA’ Boards, and Points-Sets are all identified by the abbreviation of the line to which they are associated, together with a position number that corresponds to their position in centimetres, either from the ‘0’ zero marker under the lighthouse on the jetty as is the case with the Northbound and Southbound Main Lines, or in the case of the East Chord Line from the toe of the points at the start of that line near the car-dumper. In areas where bi-directional or reversible working is in use, the direction of the movement to which the Signal or ‘POSA’ Board applies is supplemented in brackets immediately after the position number, e.g. (N) for northbound movements or (S) for southbound movements.

As can be seen from the diagram, the Northbound Main Line is a straight-forward affair, in that there is a Yard Exit Signal at Coolamusta at the NB1139 centimetre location, and a ‘POSA’ Board at the NB737 centimetre location at the entrance of the yard at Harrison Point – The case of the Southbound Main Line however is much more complex, in that this line incorporates two Yard Exit Signals at Harrison Point, i.e. Signal SB771(S) at the yard’s main exit, and Signal EC180 on the East Chord Line. Signal SB771(S) provides for main line traffic leaving the yard directly for the mine only, whereas Signal EC180 provides not only for access to the Southbound Main Line by either works or special trains, but also for ‘turning’ movements via the uni-directional ‘Turnback’ Line. It will now become obvious that the portion of line between Points-Set SB951 (where the ‘Turnback’ and Southbound Main Lines converge), and the Yard Exit Signal SB771(S) is reversible, and so two ‘POSA’ boards are required, the first being at the SB1158 centimetre location at Coolamusta Mine, and the second one for the ‘turning’ movements, with the POSA Board at SB771(N) being placed back-to-back with the Yard Exit Signal SB771(S).

To prevent two trains from being in the Southbound Section at the same time, and together with the associated risk of a flank collision that this could cause at Points-Set SB951, it is essential that Signals SB771(S) and EC180 cannot both display a ‘Proceed’ Aspect at the same time, and that to end, this is where Points-Set EC180 comes into play. The ‘Normal’ lie of this Points-Set is when they are set facing away from the ‘Turnback’ Line towards the West Chord Line and Engine Terminal, and, with them in this position, Signal EC180 will be maintained at ‘Danger’ with its associated train detector disconnected, as any passing train will not be proceeding towards the ‘Turnback’ Line. However, when Points-Set EC180 is set to the ‘Reverse’ position facing the ‘Turnback’ Line, Signal SB771(S) at the main exit will be immediately placed to ‘Danger’, and the Train Detector and Signal at EC180 will be brought into play. The Train Detector at Signal SB771(S) will however remain in an active state, because should any train pass that Signal at ‘Danger’, it will immediately place Signal EC180 to ‘Danger to prevent any further movement towards Points–Set SB951.

Things now get quite complicated for ‘turning’ movements from Signal EC180 via the ‘Turnback’ Line, and it will be noticed that in order to facilitate this move, trains must be able to pass in both directions over the Detector at SB771 - So how does the system change the Detector at SB771 from being an ‘Entrance’ Detector into an ‘Exit’ Detector? - Well the answer lies in Points-Set SB951. The ‘Normal’ lie of this Points-Set provides a route from Signal SB771(S) at the main exit of Harrison Point Yard to the ‘POSA’ Board SB1158 at Coolamusta, and it will immediately become obvious that any movement from Signal EC180 to the Southbound Main Line will involve Points-Set SB951 having to be changed from the ‘Normal’ position to the ‘Reverse’ position. When a ‘turning’ movement has come to a stand on the south side of the Position Board at SB951(N), Points-Set SB951 will have to be reset to the ‘Normal’ position whilst the section is occupied in order for the movement to continue. It is this action in this situation which tells the system to change the Detector at SB771 from being an ‘Entrance’ one into an ‘Exit’ one, whilst still providing an ‘Exit’ option at Detector SB1158. The final chapter in this cycle of events is, that after the ‘turning’ movement has been deemed to have been completed and is now back inside the yard at ‘POSA’ Board SB771(N) or has indeed gone through the section and is now clear at ‘POSA’ Board SB1158, the system will again automatically change Detector SB771 back into its original state, and clear either Signal SB711(S) or Signal EC180 depending on the position of Points-Set EC180, ready for the next train.


The most reliable method of detecting the passage of trains on railways of this type, is through the use of specially made rail-mounted treadles.

Now, most treadles simply consist of a simple rail mounted plunger, whose electrical contacts are only momentarily made or broken as each wheel-set passes over them. Given the nature of the circuitry within the ‘company’s’ C.T.C. machine, and due to the fact that it uses ‘Slugging’ Circuits which need more than a moment’s burst of current to activate them, what was needed here was a design of treadle that had a more positive action, - and so enter the ‘company’s’ own ‘Longitudinal Treadle’!

The device simply consists of a flat-mounted sprung sub-miniature single pole double throw micro-switch (Maplin Part No.  ), and a 25mm (1 inch) length of 6mm brass angle. The angle-piece is fitted to a longitudinally mounted fulcrum, which is set in a frame so that it can rock. The whole assembly is then fitted so that when the outward side of the angle-piece is flat, it is also level and within 0.5mm from and alongside the surface of the rail for the whole of its length. In this position the other side of the angle-piece is set so that it hangs downwards at 90 degrees with its face resting against the plunger of a flat-mounted sub-micro switch that is mounted next to it. The resulting prolonged action of any wheel flange passing over the assembly is such that the ‘bell-crank’ type action produced will always depress the plunger of the flat-mounted micro-switch for a sufficient amount of time, so as to activate its associated equipment back at the C.T.C. machine. The flat mounting of the sub-micro switch in this way assists with the prevention of the ingress of rainwater into the switch, and a rebound plate, which is fitted over the horizontal plate of the treadle to cancel out any rebound caused by the release of the micro-switch’s plunger, further helps to reduce this

Treadles: Parts and Method of Assembly



A fully assembled treadle: Located, and complete with its protective cover


The level crossing on the Turnback Line at Gricer's Bridge, is fitted with working amber and alternate red flashing lights as well as a 'Yodalarm' sounder. This sounder sounds only for first three seconds of the crossing's operation whilst the steady amber warning light precedes the illumination of the red lights - To prevent any continued annoyance to the neighbours through the repeated sound of the sounder going off, the crossing stays operational continuously whilst Points EC180 are set in the 'Reverse' (Turnout) position towards the Turnback Line.



Left: The 'George Cross' Sign which is provided to warn drivers of the level crossing immediately ahead on the Turnback Line to the right. The box next to the points (EC180) contains the 'Reverse' position detection contacts which operate the crossing - Right: A view of the level crossing as seen looking towards the steps from the inside area of the Turnback Line.