The railway is equipped with a 'DIGITRAX' Digital Command Control system for traction purposes, with twin access points for the portable hand controllers being provided at Crompton Junction Station, Crompton Junction South, and also at Yeominster Town, which is also happens to be right next to the landward end of the pier at West Hallowes.

All portable hand controllers can work any traction unit directly from any access point, anywhere on the layout.

The DCC equipment originally comprised of a DIGITRAX 'Empire Builder' set, but now uses additional UT4 hand controllers instead of the original DT402 master unit, that unit now being used solely by Millfield Workshops on their DCC testbench.

The DCC equipment was purchased from DCC SUPPLIES in Worcester (

The signals and points on the layout are totally analogue, and are worked by a seperate relay interlocking machine which has now become known as the 'AUTOMATIC BOBBY' (see below)!





Introducing the 'AUTOMATIC BOBBY'


Past experience has found that the usual extensive and complicated control panels are nothing short of downright baffling for the average visitor, and that extensive training is needed before any decent standard of operating is achieved..... To that end it was decided to take the 'signalman's role out of the operator's hands, and to provide what is in effect an automatic signalman that the operators tell where they want their trains to go on the layout......

The 'AUTOMATIC BOBBY' is basically a rack full of relays, as would be found in most prototype electrical interlocking machines on the full sized railway. These relays determine which routes are available to be used for the safe passage of trains, and which aren't. The information required is provided by the electrical circuits that tell the interlocking machine the state of the occupancy of the track ahead, and of any conflicting routes that may be set across the route requiring to be called - The machine is fitted with a degree of preselection, which means that, whilst any route is in use, any conflicting route subsequently called will be held in a queue until the first route has been cancelled out either manually, or automatically by the passage of the train ahead.

To enable the system to function correctly, full train detection is used, and this is achieved through the use of two methods of train detection, 'EDOT' (Electronic Detection of Trains) for detecting the position of traction units on sections of track called track circuits, and IRDOT' (Infra-Red Detection Of Trains) for detecting both traction units and rolling stock at specific points on the railway. The former method of detection is done by specially homemade 'EDOT' units on the interlocking machine, which will be described shortly, while the second method is achieved by strategically placed IRDOT2D units purchased from HEATHCOTE ELECTRONICS of Stoke-On-Trent ( These IRDOT2D units, which are fitted under the actual track on the baseboard, each send up an infra-red beam up to the vehicles that are passing overhead. When a unit receives a reflection of its own beam, it sends the relevant information back to the interlocking machine, which in turn will apply the relevant conditions of interlocking within the machine itself.

As a train proceeds across the junctions within a route, the operation of the track circuits and IRDOTs will hold the route in the 'SET' state until the interlocking machine has detected that all track circuits and IRDOTs within the junction are clear again, after which, it will then automatically Cancel-Out the route behind the train, set the protecting signal to 'ON' (Danger) and restore all the points sets to their 'NORMAL' positions.


Four views showing the 'AUTOMATIC BOBBY'... The whole assembly is hinged and supported on a heavy-duty castor wheel - It therefore opens like a door to enable easy access to the rear for repair and maintainance purposes.....

Top view, showing a close up of the ducting and wiring to the main relays - Left view, showing the front of the machine - Right view, showing the rear of the machine. Note the main power supply cover and DIGITRAX DCC command unit on the centre row. Diode and capacitor racks at the top, and main relays and another diode rack at the bottom - Bottom view, shows the 6-Pole 'R10' relays in their sockets at the bottom, and some of the EDOT units at the very top left.



controls and indications


Each starting signal is provided with a green 'Train Ready to Start' (TRST) button, and every shunting signal is provided with a black 'Train Ready to Start' (TRST) button. Each route on each signal has its own TRTS button. Each signal is also provided with a red 'Cancel' button - These buttons are all set under the side of the baseboard next to the signals concerned.

Above each set of buttons are two sets of indicator lamps. A red one to show when the signal concerned is showing "ON' (or Danger), and a white stencilled letter 'F' (FREE) to show the condition of each route that is available from it. If a route, after being called by the operator cannot be called because a previous conflicting route has been called and set first, the 'FREE' lamp will continually flash to show that it is in the queue on the interlocking machine. When the route is eventually called by the interlocking machine, the 'FREE' lamp will become steady, and the interlocking machine will then set the points in the route and any points protecting the route accordingly. When the points have been detected to be in the correct position, and the track is still proved to be clear to the next signal, the signal concerned will clear and the red light next to the TRTS button will be extinguished.


Crompton Junction 'TRTS' and 'Cancelling' controls - Up side signals on the left, down side signals on the right ........... Signals 120 and 127 are showing a 'Proceed' aspect, with No 127 being in 'Automatic' mode - Note the DIGITRAX access sockets on the right.


The West Hallowes and Yeominster 'TRTS' and 'Cancelling' controls are situated side by side due to the close proximity of both locations - The black buttons between the red and green buttons of the West Hallowes signal controls are for use when shunting from one platform to another at that station. If pressed, these buttons ensure that the route from the next signal ahead at Crompton Junction South will not automatically be called - Note the DIGITRAX UT4 hand controllers on the right that are plugged into the access sockets below....


The Main Line signals protecting the stations at both Crompton Junction and West Hallowes are all designed to work automatically, with the one on the Up Southern Line at Crompton Junction South being fitted with an additional 'AUTO WORKING' over-ride switch.



additional controls


The Western Region line to Yeominster is worked differently from the Southern Lines, in that it's line occupancy system is set to a 'One-In-One-Out' basis..... But, two local control panels, one at Yeominster North, and one at Yeominster South, allow access to and from the run-round loop and test track at Yeominster - Also at Yeominster, it is possible to let a second train down to the station from Crompton Junction after the first one is clear of the Up and Down Western Line and locked in on the Test Track. This is achieved by the operator of that train restoring all the local points at Yeominster to 'NORMAL' and pressing an additional 'RESET' plunger on the Yeominster North Local Control Panel.


The Yeominster North Local Controls - Note the 'Reset' and 'Train Clear of Single Line plungers........







All points, including the locally controlled ones at Yeominster, are motorised with COBALT slow-acting motors as purchased from TRACK SHACK on the Isle of Man (

Well designed, robust, and easy to fit, these strong little motors come complete with proving contacts - A must for every modeller who uses electricty to change their railway's points!





Colour light signal heads were all supplied by the former company of 7mm.Co, now ROUTEMEX of Stirling (, and all mechanical signal dolls were supplied by SCALE SIGNAL SUPPLY through INVERTRAIN of Dunfirmline (

Many signals will be fitted to scratchbuilt gantries and other structures, and all mechanical signals except the Up Western Distant Signal for Yeominster Town (which will be fixed at Caution) will be worked by motors and electronic controls as supplied by G.F. CONTROLS of Wigan (





Many readers of this website will have at one time or another contemplated about how they could possibly show the position of their trains on their control panel diagrams in real time, without the need to resort to the use of a combination of expensive DCC equipment and computer software. The aim of this would be not only to bring about the possibility of introducing the role of Signaller/Line Controller/Dispatcher into the operation of the layout concerned, but it could also bring some added protection to those valuable locos and other treasured items of rolling stock that are usually in use, through the provision of additional interlocking - Well, as an ex-signaller myself, I wanted to do just that, and that was way back in the early 1980s!

Some of our ‘older’ readers, may remember a layout called ‘FISHER STREET-VICTORIA BRIDGE’, a ‘00’ scale ‘Woodhead’ orientated layout which appeared in the Railway Modeller in August 1985. When this layout was rebuilt in ‘0’ scale in 1988, I took the opportunity to build a working miniature lever frame complete with mechanical interlocking, and to install an associated Train Detection System in order to work it.

Types of Train Detection
Now, there are basically two types of train detection, or ‘Line Occupancy’ systems, as they are known, in use in the model railway fraternity:

The first type of Line Occupancy system is called the ‘LATCH-IN/LATCH-OUT’ system – This system is basically a relay, which, as a train enters its section, is powered-on and instantly ‘latched’ over on itself into a continuously self-energised state. It is then de-energised or ‘released’ again as the train leaves the section at the other end. These activations are usually achieved by using either, Reed-Relays, which are buried under the track and activated by the magnetic field belonging to any electric motor which just happens to be passing overhead, or, by rail-mounted switches, or ‘Treadles’ as they are known, that are depressed by the action of the train’s wheel flanges as they pass over them, or finally, by using such intricate things like the Infra-Red Detection Of Trains Units, or ‘IRDOT’s as they are known, which can be obtained relatively inexpensively from firms like HEATHCOTE ELECTRONICS ( The main drawback with ‘IRDOT’ Units compared to Reed-Relays and Treadles, is that each one has to be mounted under the baseboard with its associated transmission and receiving probes ‘looking upwards’ through two small holes that have to be drilled through the baseboard in between the running rails where each section meets. However, ‘IRDOT’s do have one major advantage over Reed-Relays and Treadles though, and that is that they will detect when any vehicle is located above them at all times – To conclude then, it is obvious that with any form of ‘Latch-In/Latch-Out’ system, a Reed-Relay, Treadle, or ‘IRDOT’ Unit will have to be provided at the entrance of each track section concerned, and another one will have to be provided at the exit end.

The second type of Line Occupancy system, which I am going to describe here, is the ELECTRONIC DETECTION OF TRAINS system, or ‘EDOT’. This system electronically detects the presence of any electric motor, which is connected across the running rails that forms a section of track section that is known as a ‘Track Circuit’.
The electronic circuits used here will not only work with any Conventional DC equipment (except PWM Controllers), but they will also work with DCC Digital Command Units, and, if you manage to build some of these Detection Units yourself (I have actually built seventeen of them), you will find that even today without the integral relay, the cost of each individual Unit still comes in at under a ‘tenner’!
However, before we go any further, I must now draw your attention to the one major drawback that both the ‘LATCH-IN/LATCH-OUT’ and the ‘EDOT’ systems possess, and, that is that due to the fact that in all cases it is only the leading wheels, and/or the powered vehicle that is being detected, and not the whole train itself, you will have to use what is known in the industry as a ‘Double Block’ method of working. This is where the first train has to be three sections ahead before a second train can be allowed to follow – However again, if you decide to do as I have done on this layout, and use a combination of both ‘EDOT’ and ‘IRDOT’ units for each individual Track Circuit, the system will prove reliably as soon as possible when each complete train, and not just the detected vehicles, has passed clear of any individual Track Circuits and junctions etc.……
So now, here are details of the ‘EDOT’ Units that I built way back in the 80s, together with the instructions on how to wire them in.
Again, before we go any further, due to the nature of the power supply requirements, these ‘EDOT’ Units are not suitable for use with PWM controllers!


Left: Home made EDOT Unit as built in the 1980s - Right: Under-baseboard IRDOT2D Unit, as supplied by HEATHCOTE ELECTRONICS ( Note the infra-red transmitter and receiver probes that poke up through the baseboards between the running rails

Power Supplies, and Wiring
Each Unit consists of two sections, a Detector and an Amplifier. It is placed in-circuit, or ‘in series’ as it is known, between the Return Rail of each Track Circuit via Terminal ‘C’ on each Unit, and the layout’s Common Return wiring via Terminal ‘E’ on each Unit. It is discretionary whether or not the layout’s Common Return wiring is connected to Earth or not! – In all cases a totally separate 12vDC supply must be provided via Terminal ‘B’ on each Unit to work the Amplifier section, with the negative side of that supply also being connected to the Common Return. If the Unit is going to be provided with an integral relay as mine are, then the 12vDC supply must be ‘smoothed’ to avoid any resulting ‘chatter’ that will soon damage the relay! - Where the Units are going to be used with Conventional DC equipment, an additional 12vAC supply (see below) will be required in addition to the above. One side of this supply must also be connected to the Common Return.

'EDOT' Unit - position in circuit

Due to the fact that the metal plate, which forms the heat sink in each Unit, plays a part in the electronic circuitry, each Unit must be mounted separately onto a non-conductive surface.

The Detector Section
The Detector Section of each Unit consists of two 2N3055 115Watt NPN transistors, both of which are directly mounted onto a common heat sink. This heat sink acts as a physical connection between the two bodies, which form the ‘Collector’ terminals of each respective transistor, and it also provides a means of connecting the Detector Section to the Amplifier Section of each Unit via a ‘Sensitivity’ Resistance (see below).
When any powered vehicle, or any other piece of rolling stock with resistive wheel-sets is standing on the running rails that forms a Unit’s associated Track Circuit, the circuit will be completed, and the detection of the vehicle’s presence will take place as follows:
Terminal ‘C’ of the Unit (the connection for the Return Rail of the Track Circuit) is connected both to the ‘Base’ terminal of the first transistor and to the ‘Emitter’ terminal of the second transistor. The ‘Emitter’ terminal of the first transistor in turn is connected to both the ‘Base’ terminal of the second transistor and to Terminal ‘E’ of the Unit (the connection for the layout’s Common Return wiring). This part of the circuit forms a path of minimum of resistance for the Traction Current to flow when the powered vehicle is travelling in either direction, whilst also ensuring that the powered vehicle, or indeed any other piece of rolling stock with resistive wheel-sets continues to be detected at all times when stationary, as I will now explain…….
In order to detect when any powered vehicle, or any other piece of rolling stock with resistive wheel-sets is completing or ‘occupying’ a Track Circuit, the detecting Unit will require a constant AC voltage to be applied across its ‘C’ and ‘E’ terminals in order for it to work.
It must be remembered that when using DCC equipment, the rails are continually energised in this manner, but when using Conventional DC equipment, they are not, and so to combat this, a separate and continuous, but non-destructive AC voltage must be introduced to all of the Feed Rails on the layout. This is done by using a set of 3.3uF 25v ‘Reversolytic’ Capacitors that are each connected to the 12vAC supply on one side, with the other side of each one being independently connected directly to each of the Feed Rails that form the isolatable sections. Finally, it must be remembered that one last ‘Reversolytic’ Capacitor will be required to connect the 12vAC supply to the remaining Feed Rails that form any non-isolatable parts of the layout.
Unlike ordinary electrolytic capacitors, ‘Reversolytic’ Capacitors are designed to safely work in any AC environment, and, as they are not ‘polarised’, they can be wired either way round, and are not affected by the changes of polarity which occurs in the Traction Voltage when a change of direction is selected for the powered vehicle involved. Their purpose is twofold. The first is that, they will act as an AC resistance, or ‘impedance’ as it is known, in this case presenting an impedance of about 1000ohms to the 12vAC supply at 50Hz, and so only allowing a harmless 12mA to pass through any detected motor when stationary. The second purpose is that they will also block any DC Traction current from entering the wrong side of any adjacent isolating switch circuitry in the control panel.
When any powered vehicle, or any other piece of rolling stock with resistive wheel-sets occupies a Track Circuit belonging to a Unit, the action of the electric current flowing between the ‘Base’ and ‘Emitter’ terminals of each of the Detector transistors within the Unit, will in turn produce a voltage on the heat-sink of the unit concerned – This voltage is then sent through a ‘Sensitivity’ Resistance to the Amplifier Section (see below).

The Amplifier Section
This is a single stage amplifier, which can be regarded for the purposes shown here simply as an electronic switch. The condition of the output of this Amplifier when any powered vehicle, or any other piece of rolling stock with resistive wheel-sets has not been detected is, ‘ON’ - When any powered vehicle, or any other piece of rolling stock with resistive wheel-sets has been detected, a small voltage will be received through the ‘Sensitivity’ Resistance, directly to the ‘Base’ terminal of the Amplifier’s Transistor. This will in turn set the output of the Amplifier to, ‘OFF’!
In my units, each Amplifier drives a small integral Track Circuit Relay, or ‘TR’ as it is known in railway parlance. This ‘TR’ has a ‘Slow-To-Pick/Slow-To-Release’ property, which is activated by the 100uF 25v Electrolytic Capacitor that is placed in ‘parallel’ across the Amplifier Transistor’s 2k2 Biasing Resistor. Again, in railway parlance, the ‘TR’ in each of my Units is ‘Slugged’. This ensures that if any intermittent loss of detection is encountered, the ‘TR’ will not be quickly re-energised or ‘Picked’ as it is known, and any other indications will not ‘bob’ between showing ‘Occupied’ and ‘Clear’.

Each of my Unit’s ‘TR’s, through their own independent contacts, are in turn connected to my 120-Relay Signal Interlocking via the other three terminals on each unit. These terminals are as follows:

Terminal ‘A’: ‘Armature’ (the moving contact in the relay - In railway parlance, this is also known as the ‘Arm’ of the relay)

Terminal ‘N’: ‘Normal’ (the normally de-energised contact in the relay - In railway parlance, this is also known as the ‘Back’ Contact of the relay)

Terminal ‘R’: ‘Reverse’ (the energised contact in the relay - In railway parlance, this is known as the ‘Front’ Contact of the relay)

To conserve space on each Unit, I have used a small 2N3705 NPN transistor in the Amplifier Section - This type of transistor along with its equivalent BC107/108/109 types, is suitable for powering small items such as relay coils or Light Emitting Diodes, but should any larger incandescent types of bulb be used for indication purposes, then perhaps a further 2N3055 type (as used in the Detector Section) should be employed instead.

Detection Sensitivity
This is a matter, about which I have had to learn about the hard way over the years………….
As I stated earlier, I first used these units successfully on my first ‘0’ scale layout way back in 1988. Now, this layout was actually situated indoors, but the building that I was occupying at the time was soon to be commandeered for use as a photographic studio in connection with my then new business. The result of this was that I had to self-eject both my model railway activities and myself out of this building and into the outside world, i.e. the garden! A new garden railway portraying the then current scene of the North-West Australian Iron-Ore operations was constructed shortly afterwards, and this railway incorporated a working level crossing in order to protect my trains from the users of a footpath! Now, this level crossing was actually fitted with flashing red lights and ‘Yodelarms’ (yes – “#ee-or-ee-or-ee-or…..#”), and it was designed so that when any locomotives occupied the Track Circuits that were associated with it, it was activated. So, you can now imagine the response I got, when shortly after it was commissioned, my then thirteen year-old daughter was awoken at 8.30am (the middle of the night to her at the time) by the crossing ‘self-activating’ outside her window… Needless to say, I got plenty of ‘grief’ for her disturbance, and further investigations revealed that there had been a sharp downpour shortly before the event. Thank God it didn’t happen at 4.30 in the morning, as I think that the neighbours would have had something to say! – Basically, the units were oversensitive, and further investigations concluded that if any moisture were detected between the running rails, the associated Unit would instantly think that a train was present when it actually wasn’t the case - In railway parlance, this type of fault is known as a ‘RIGHT-SIDE (failed safe) TRACK CIRCUIT FAILURE’!
To overcome this problem, a 330ohm ‘Sensativity’ Resistor has now been installed between the output of the Detector and the input of the Amplifier Sections in each Unit. This value has been arrived at by using the long-tested and proven ‘trial-and-error’ method, but the general rule seems to be that, the lower the resistance between the output of the Detector Section and the input of the Amplifier Section, the more sensitive the Unit will be. Having learned this all for myself, I would now advise that some form of ‘Sensitivity’ adjustment in the form of a 1k variable resistor or potentiometer is fitted in lieu of my ‘Sensativity’ Resistor on all new builds.






A wall mounted diagram has been provided to show the position of all trains, along with the condition and lie of all points and local controls on the whole of the railway.......