Simple Signaling for Common Rail Two Cab Wiring


Block assignment and Turnout position indication can prove to be amongst the biggest stumbling blocks when trying to operate a new Railroad or when training new operators on an existing model Railroad. Bicolor LEDís placed between the rails are a good indication, but un-prototypic. Lights on the fascia are useful, but have to be read in conjunction with the toggle position to provide accurate information. The ultimate solution is an interlocked signaling system, ether on the panel, the layout or preferably both.

Working Signals have long been the dream of many a model railroader. In the absence of motive power, nothing gives more life to a railroad, real or modeled, than the sight of signals changing. But this dream is often considered as well beyond the capabilities of the average model railroader, due to ether cost, complexity or both. With this in mind I have developed a system that requires only easy to use components, namely resistors and low voltage bulbs or LEDís, that any one capable of wiring a model railroad will have no problem installing. It is a passive system, so no occupancy detectors are required and no modifications need be made to your rolling stock. I also wanted the system to be able to fit in with existing model railroad wiring, and easy to implement on new model railroads.


The system was conceived, primarily, to provide an indication of the selected route for a cab. The indicator lights could then be mounted on signal heads on the layout, control panels along with the switches or both. This will assist the operator to see if the selected route is useable, and at the same time provide interesting animation for the casual viewer (one ounce of work for a pound of pleasure).


For the purists amongst us, I must point out that this is a very simplistic (although useful and effective) signaling system, with no known similarity to any prototype. It provides two aspects: Red (Stop) and Green (Proceed). I am sure that it will satisfy most modelers, and many operators for the valuable feedback it provides. It will also assist newcomers to work with twin cab control (rule no1 never run a red light).

The Basics

To implement this system a basic knowledge of model railroad wiring is required, so a bit of background is in order. Until the advent of the NMRA standards for DCC, two-cab common rail wiring was (perhaps undisputedly) the most versatile method of wiring a railroad for operations, and it is this form of wiring that provides the necessary blocks for signaling. Two Cab wiring requires that the railroad be separated into control blocks, with the power being fed through a single-pole double throw (SPDT) switch, and the return through a common rail. (As this is not the place to fully describe two cab wiring, I refer any one who requires more in depth information to the book, Easy Model Railroad Wiring, Andy Sperandeo, Kalmbach Books, ISBN 0-89024-349-2). Many people have used double-pole double-throw (DPDT) switches instead of SPDT ether because of availability or to use the extra contacts to indicate that the block is in use. It is this extra set of contacts that provides the basis of the Simple Signal System.


The first component of the Simple Signal System is the power supply, which is required to provide v+, v- and a common connection. This can be derived for ether an AC or DC source (DC canít be used if you plan to have LEDís as indicators).



In the same manner that the DC outputs of your power packs can be connected to produce a common connection, we can do the same with the Accessory outputs to provide power for our signals, as in Fig. 1. I would, however, recommend that a dedicated center tap transformer be provided for this application, to avoid short circuits and possible damage to expensive locomotive motors. The total number of bulbs or signals that you will require determines the total current rateing of this supply.


Fig 2 shows the logic circuit required for each signal. The circuit uses a combination of positive and negative logic to provide an exclusive OR function for the green aspect with the default aspect being Red. The Resistor values will also be determined by the type of indicators (bulbs or LEDís) and the supply voltage, some experimentation may me required to find the optimum brightness in your situation. For the sake of this article it is sufficient to know that one floating input or two like inputs will provide a Red indication and opposing inputs Green.


In use, a green indication (fig 3a or fig 3b) shows that two consecutive blocks are connected to the same Cab and the turnout is aligned, so the train may run through the block boundary nonstop. A red indication (Fig 3c) shows that the two blocks are set for opposing (or unselected) Cabs or that a turnout is thrown against the train. If all signals are dark, no route has been selected. This indication may happen with center off type block switches. (This is useful for testing. If any one block is selected at least one signal should show red).

Connecting it up


The inputs to the signal driver are marked as E (east) and W (west). A common connection is also needed. The signals/indicators are located at the block boundaries, and receive input from the block selectors. East is the proceeding block and West is the following block. Turnouts are not considered as separate blocks, but rather as part of the block containing the points and preceding the frog of the turnout.


As can be seen from the basic wiring diagram (Fig 4), connections from the DPDT switch, to the signal driver, are similar to those for the track. It is important to note, however, that when Cab A is selected on the East block it feeds V- into E, but if Cab A is selected on the West block it feeds V+ into W. This is the only tricky part of the wiring, and is easy to detect. If one or the other connections are reversed the circuit will not be damaged, but will give a red indication for a valid route and green for opposing routes.


Fig 5 shows a more complex arrangement, containing 4 Blocks and 2 Turnouts to form a passing siding.



Careful study of the diagram will show how the circuit works, paying special attention to the alternating polarities of the blocks. Note also the connections of the turnout indicator switches. The turnout switches are placed in series with the block switches and as such determine which driver is selected. Any spare SPDT contacts on a switch machine or ground throw may be used. Alternatively, you may control your turnouts with the ďwire in tubeĒ method, via a DPDT slider, which provides the necessary contacts. If you prefer, or do not have the necessary contacts, connect B1 to S1E and S2E, likewise connect B3 to S3W and S4W. The signals will then indicate valid block selections, but the Engineer will have to look out for the proper turnout positions.



This system is only slightly more complex to wire than normal control panel indicators, but provides vastly more information and functionality, especially if the block control switches are not of the center off verity. In fact the difference in price of DPDT Center off and standard DPDT switches is probably enough justification for any one starting with Cab Control to adopt this approach to block indication. No PCB diagrams are provided, as most users will find it easier to solder the resistors in line with the Bulbs/LEDís and enclose it all in insulated tubing.



This system is theoretical at this time, as I am still building my layout ( and have not tested the theory in practice. If you have any comments on the theory or wish to try the system out I would welcome your feedback (good or bad) at I retain the rights to this design but accept no responsibility for damages should you implement the idea. Thanks for reading this far.




Chris Roper

Hout Bay,

South Africa.