Information on ordering a commercial kit or assembled and tested unit of these circuits is available from the TracTronics Price List.
This is the eleventh in our series of articles describing a set of electronic building blocks we have designed to control our model railroad layouts: Rich Weyand's N&W Pocahontas Division, Bill Pistello's Union Pacific, and the Reid brothers' Cumberland Valley System. This month we will finish our discussion of signaling. We previously discussed Automatic Block Signaling (ABS) and Absolute-Permissive Block (APB) signaling, and described circuits to implement these systems. This month we will discuss the most complex signaling scheme of all, Centralized Train Control with intermediate ABS signals and fleet mode.
Centralized Train Control, or CTC, is a term applied to a wide variety of signal systems. What they all have in common is that train movements are controlled via trackside signals, which in turn are controlled by signal operators at one or more locations. The signals are always interlocked so that an operator error cannot result in two trains having permission to operate over the same track at the same time. A signal system on a prototype railroad typically has dozens of different signal indications for different circumstances. The variety of indications is confusing enough that professional train crews carry guides explaining all the different possible indications they might see. The distributed array of controlling electronics for the signal system itself is complicated beyond belief.
In the April 1994 Mainline Modeler we described a simple CTC signal system for model railroads using the BlockLockTM circuit. This is the system we installed on Bill and Wayne Reids' Cumberland Valley System for their Pennsylvania Railroad branch line from Harrisburg, PA to Hagerstown, MD and on to Winchester, WV, set in the year 1953. Those of us modeling the modern era, or main lines of Class A railroads, wanted a more elaborate and complete CTC system. This system had to include intermediate permissive ABS signals which would maintain spacing between trains in a single track portion, as well as fleet mode operation in which the absolute signals are set to operate in ABS mode.
We already have the tools we need to implement this system. The DetectTrainTM train detector circuits, discussed in the February 1994 Mainline Modeler, will determine occupancy of track blocks. The AutoBlockTM and AutoSearchTM signal driver circuits, discussed in the September 1994 Mainline Modeler, will drive the individual signals. The WhichWayTM direction precedence circuits, discussed in the November 1994 Mainline Modeler, will determine and remember the direction of traffic in the single track portions. Finally the BlockLock CTC panel circuits we discussed in the April 1994 Mainline Modeler will control the system and interface to the CTC panel lights and controls.
In order to implement the CTC system, we have made some modifications and additions to the circuits previously introduced. These changes are not needed to implement the systems previously described, and they make the boards a little more complex and costly to make, but they are necessary for CTC.
The DetectTrain train detector circuits are used as previously described without modification.
We have included additional stop and approach inputs on the AutoBlock and AutoSearch signal driver circuits. These circuits drive the signals, displaying red whenever one of the Stop or Direction inputs are connected to ground, yellow whenever one of the Approach inputs are connected to ground and the Stop and Direction inputs are not, and green whenever none of the inputs are connected to ground. For CTC use, we need 3 Stop and Direction inputs for red, and 3 Approach inputs for yellow. The circuit layout and component placement diagrams for the modified AutoBlock and AutoSearch circuits are shown in Figure 1 and Figure 2. The inputs are now D, S1, S2, A1, A2, and A3.
We have included additional Intermediate Block inputs on the WhichWay direction precedence circuit. This circuit determines which way the train was going when it entered the single track portion and remembers it until all of the track blocks are empty. Unlike the APB signaling system, the CTC panel controls will determine the direction through the single track portion, and so we need two additional IB inputs for the OS sections, to insure that the signals do not clear until the train has cleared the fouling points of the switches at either end of the single track portion. We also need three additional outputs, the inverted sense of the EAST and WEST outputs, to interlock the CTC panel circuits, and a combined occupancy output, to drive a CTC panel occupancy light when there are trains anywhere in the single track portion. The circuit layout and component placement diagrams for the modified WhichWay circuit are shown in Figure 3. The inputs are now EOS, IB1, IB2, IB3, IB4, IB5, IB6, and WOS, and the outputs are now EAST (E), WEST(W), NOTEAST(~E), NOTWEST(~W), and OCCUP. The OCCUP output is the unlabeled output in the middle of the right side of the board layout. There was no room to fit the label.
The BlockLock CTC panel circuits should be constructed for use in this CTC system by substituting jumpers for resistors R5 through R12. No other modifications are necessary.
The parts list for the modified AutoBlock, AutoSearch, and WhichWay circuits are given in Tables 1, 2, and 3. In addition to the extra components needed to support the additional inputs and outputs, we have changed the values of resistors R1 and R4 on the AutoBlock circuit to increase the noise margins of the circuit.
Figure 4 shows the wiring for CTC signaling for a stretch of single track with three intermediate track blocks. Figure 4 has been set up to overlap with itself to form a signal wiring diagram for your layout. Make multiple copies of Figure 4, and cut them off along the dotted lines at either side of the figure. If you now butt the figures together, you can build up the diagram for the system you need.
While the circuit of Figure 4 looks extremely intimidating, its operation is pretty simple, and we'll walk through it one step at a time. You should first note that Figure 4 is very similar to the diagram we used last time for APB signaling, and to the diagram we used the time before for ABS signaling. These signaling system diagrams are a progression, each building on the last, and can be best understood by following the changes from one to the next.
As before, the left to right, or eastbound, signals and signal driver circuits are on the bottom, and the right to left, or westbound, signals and signal driver circuits are on the top. Only one set of detectors is required. The positive supply voltages provided to the detectors, the signal driver circuits, the direction precedence circuit, and the CTC panel circuits need not be the same, or even from the same power supply, but the grounds of the supplies must be connected together. Of course one supply can also be used. Again, the RLED limiting resistors are probably not necessary, but are shown in case they are required for your application.
As in APB, the OCCUP signals from the track block detectors are connected to the two westbound signal driver circuits located east of the track block, and to the two eastbound signal driver circuits located west of the track block. The OCCUP signals from the detectors in the single track portion, including those of the OS sections, are also connected to the direction precedence circuit. Recall that OS means "over switch" and refers to the switch and the tracks into the switch from the fouling point of both branches through the switch points to the absolute signal.
The last and second last signals on the single track section are wired as with APB, to display yellows when the next single track section has oncoming traffic. Finally, as in APB, the detectors of the OS section are wired to the detector of the main line through the double track portion with 1N4001 diodes (Radio Shack 276-1653) so that the OS sections cooperate in the spacing of trains.
There are three primary differences in this circuit from the APB circuit. The first is that the signals have been somewhat rearranged for CTC. The locations of the signals along the main in the CTC system are shown in Figure 5. Note that the signals at the point where the double track goes to single track are no longer across the single track main from each other, but are on either end of the OS section. These are absolute signals, and will always be marked as absolute signals, either with an "A" marker sign, or with a marker signal. You should research the practices of your prototype railroad to find whether they used a marker sign or a marker signal, and where on the mast the marker signal was located. We have not included the marker signal in the wiring diagrams, as it should be wired to always display a red signal.
We have added a separate absolute signal for trains on the auxiliary main, or siding, at the entrance to the single track portion in each direction. This signal is a dwarf signal, called the B head, mounted on a short mast to distinguish it from the main signal, called the A head. We have also added another head to the signal mast at the entrance to the double track portion in each direction. The upper signal head, called the A head, is for the path through the switch onto the main track, and the lower head, called the B head, is for the path through the switch onto the diverging route, which here is the auxiliary main, or siding, track. The A head and the B head at each location are considered one signal, and are given a single signal number.
You should notice that some of the signals are shown on the fireman's side of the track. These signals, if placed on the engineer's side, would be between the two tracks of the double track portion. There is usually not enough clearance for these signals between the tracks, and so they are often installed on the fireman's side. Any of these signals can also be mounted on overhead cantilevers or signal bridges, of course, in which case they will be mounted directly above the center of the track to which they correspond.
The second change is that we have added the CTC panel circuits to the system. These circuits control the EOS and WOS inputs to the direction precedence circuit, as well as the direction inputs of the signal driver circuits of the eight absolute signal heads. The new NOTEAST and NOTWEST outputs of the direction precedence circuit also connect to the CTC panel controls, to interlock the signals at each end of the single track section. They are connected to a resistor to +5 volts (3.3 kohms, 1/4 watt; Radio Shack 271-1328) and a capacitor to ground (0.1 uf, 15 volts; Radio Shack 272-135) at the CTC panel to suppress noise in the long cable run to the CTC panel. The new OCCUP output of the direction precedence circuit powers the occupancy lights of the CTC panel. The eleven resistors on the LEDs at the bottom of Figure 4 are all 150 ohm, 1/4 watt (Radio Shack 271-1312) and the six diodes are all 1N4001 (Radio Shack 276-1653)
The third change is the addition of switch circuits to the wiring to select the active signals at each end of the single track sections. These single-pole double-throw switches, two at each end of the single track section, select whether the main or the siding will have the green into the single track section, and whether the upper head or the lower head will clear into the double track section, based on the switch position. When the switch is set to the main, these switch contacts should be in the lower position shown in the diagram, and when set to the siding, the contacts should be in the upper position. You can actually use the same single-pole double-throw switch for the two switches at either end of the diagram, using only one set of contacts on your switch machine. We drew it as two switches to avoid cluttering the diagram any further.
The CTC panel layout for the track configuration shown in Figure 5 is given by Figure 6. The track diagram at the top has five occupancy lights: one red occupancy light on each OS section, and two yellow and one red occupancy lights in the single track portion which indicate both occupancy and traffic direction as indicated by the arrows. The center red LED indicates that the track is occupied without the direction precedence having been set, such as by a loose car or an open electric lock switch. The three red and two yellow LEDs are shown in the center bottom of Figure 4 in the correct order as on the panel.
One thing you should note about the occupancy lights for the OS sections. If you equip only your cabooses with axle resistors, the detectors will only detect the loco and the caboose of each train. As the OS sections are much shorter than a train, the OS section occupancy light will go on then off as the loco moves over it, and go on then off as the caboose moves over it. The circuit works properly in all other ways, but if the OS section occupancy light going on then off twice for each train is disconcerting to your dispatcher, you might consider equipping every car with an axle resistor or drop the OS section occupancy lights from your CTC panel.
The signal indications are displayed on the CTC panel by two red and four green LEDs mounted above the signal controls. The green LEDs are shown connected to the RGP and LGP outputs of the CTC panel circuits in Figure 4, in the correct order as on the panel. The red LEDs are connected through 1N4001 diodes to the RGP and LGP outputs, so that when either green LED of one of the blocks is on, the red LED for that block is off. Note that the CTC operator has no display of the intermediate signals, and cannot tell whether the absolute signal is displaying a red stop or yellow approach indication. The CTC operator only knows whether the signal is cleared or not.
The CTC panel rotary switches are located below the OS sections they control. We use double-pole six-throw (2P6T) rotary switches (Radio Shack 275-1386). Below the rotary switches are the code buttons, for which we use mini push-buttons (Radio Shack 275-1547). Finally, at the bottom are the fleet mode switches, for which we use single-pole single-throw toggle switches (Radio Shack 275-612). These toggle switches are mounted so that they are open when the toggle is in the up position.
We have also shown the switch controls for the two mainline switches on the CTC panel layout of Figure 6. Note that the switches are given odd numbers and the signals even numbers, and that the numbers are approximately in order across the panel. Also note that the A head and the B head are given one signal number, even when they are not mounted on the same mast. This is as on the prototype.
The operation of the circuit is extremely close to the prototype. When no trains are in the single track portion or on the OS sections, and no signal has been cleared through the CTC panel, the eight absolute signal heads all display red stop indications, due to the RRL and LRL outputs of the CTC panel control circuits being low, holding the DIRECTION inputs of these signal drivers to ground. The RGL and LGL outputs of the CTC panel control circuits are high, and so the EOS and WOS inputs of the direction precedence circuit are high, and the EAST and WEST outputs of the direction precedence circuit are high, so the four permissive signals all display green proceed or yellow approach indications.
When a signal is cleared, by setting the rotary switch on the CTC panel to the left or right and pushing the code button, the corresponding RRL or LRL output of the CTC panel circuit goes high, releasing the DIRECTION input of the appropriate absolute signal driver circuit. The signal head may or may not display a green proceed indication, depending on the location of trains, but the signal has been cleared to allow a green proceed or yellow approach indication to be displayed when appropriate. If the cleared signal is into the double track portion from the single track portion, the cleared signal head will be the upper head if the switch is set for the main track, and it will be the lower head if the switch is set for the auxiliary main, or siding, track. The switch position will ground the S2 STOP input of the signal driver to display a red stop indication on the incorrect signal head. Note that the lower signal head can only display a red stop or yellow approach indication, because the A3 APPROACH input of the lower head signal driver is connected to ground.
If the cleared signal is into the single track portion from the double track portion, the cleared signal will either be the main signal or the siding signal, with the switch position grounding the S2 STOP input of the signal driver of the incorrect signal head as before. In this case, the RGL or LGL output of the CTC panel control circuit goes low, grounding the EOS or WOS input of the direction precedence circuit, and setting up the direction output EAST or WEST to go low, connecting the DIRECTION input of the oncoming permissive signal drivers to ground, and displaying red stop indications on the oncoming permissive signals. The NOTEAST or NOTWEST output of the direction precedence circuit also goes high, connecting one position of the rotary switch of the CTC panel circuit for oncoming traffic to high instead of to ground, and making it impossible to clear the absolute signals for oncoming traffic.
When the train moves onto the OS section, the OS section detector output will go low and the CTC panel circuit will return to the uncleared state, with RRL and LRL outputs going low, RGL and LGL outputs going high. The direction precedence circuit will maintain the EAST and WEST outputs in their current state, holding the oncoming permissive signals to red stop indications, and maintain the NOTEAST and NOTWEST outputs in their current state, interlocking the CTC panel circuits from clearing the absolute signals for oncoming traffic. Finally, the OCCUP output of the direction precedence circuit will go high, supplying power to the occupancy lights on the CTC panel.
After the train clears the OS section, another train can be cleared into the single track section in the same direction by recoding the CTC panel circuit. As the first train moves across the single track portion, the intermediate permissive signals will keep the following trains properly spaced by operating in ABS mode. Once all following trains have left the single track section, and all of the detectors on the single track portion and the OS sections have returned to the unoccupied state, the direction precedence circuit will reset, and both CTC panel circuits will be able to code a clear signal in either direction again. The OCCUP output of the direction precedence circuit will go low, and all of the occupancy lights on the CTC panel will go dark.
Many CTC panels have fleet mode capability. When a signal is cleared and the fleet mode is turned on, the absolute signals do not need to be recoded for following trains. In this case the signals act as ABS signals in the direction coded. This is very useful when running many trains in one direction on a single track, such as commuter trains during morning and evening rush hours.
The fleet mode switches are shown in Figure 4 just above the CTC panel control circuits. When this switch is closed, or in the down position on the CTC panel, the CTC panel control circuit acts in the normal way, being reset by the occupancy signal from the detector on the OS section. When this switch is opened, or in the up position on the CTC panel, the occupancy signal from the OS section is disconnected from the CTC panel circuit, and the CTC panel circuit will never reset, but will remain coded for a clear signal in the last direction selected. The signals will then operate in ABS mode, per the STOP and APPROACH inputs connected to the other detectors.
As we mentioned last time, the direction precedence circuit is actually set up to handle as many as four intermediate track blocks, but this is an unlikely model situation. Bear in mind that on the prototype, the blocks are usually about two train lengths long, or about two miles per block. In HO scale four intermediate blocks would be about 500 feet long! Even for short train lengths, four intermediate blocks of two train lengths each is an unusual operating bottleneck on a model layout. Figure 4 should handle most model situations.
We talked about electrically locked switches off the main line back in the November 1993 Mainline Modeler. The basic idea is that a switch off the main line into a siding or spur must interface with the signal system to ensure that through trains don't end up with green signals into a lumber yard or a chemical plant. The controls for these switches are locked, and unlocking the controls shunts the track circuit for that main line block so that the block appears occupied and signals display red stop indications.
In the CTC system of Figure 4, the electric lock switch circuit from the November 1993 Mainline Modeler article is connected to the detector output of the main line block in which the siding or spur switch is located. This will result in proper and prototype operation of the signals for all switching activities along the main line.
For other signal types, such as signals using bulbs, searchlight signals, and position light signals, use the modifications of the signal driver circuits from the ABS article in the September 1994 Mainline Modeler to modify Figure 4.
A circuit this complicated takes some getting used to in order to understand it. First you should read the article on ABS signaling in the September 1994 Mainline Modeler again, paying particular attention to Figure 7. Work through the operation as a train passes through the section of track in the figure, paying particular attention to the signal indications displayed. Then, read the article on APB signaling in the November 1994 Mainline Modeler again, paying particular attention to Figure 4. Once again, work through the operation as a train passes through the section of track in the figure, paying particular attention to the signal indications displayed. Next, read the description of the simple CTC circuit in the April 1994 Mainline Modeler again.
With that background, go back to Figure 4 in this article, and work through the process of one train passing through the section of track shown in the figure. While there are a lot of connections, the operation of this circuit is not that complicated. Once you have come to understand this circuit, you will be able to modify it for different situations you find on your own layout, and to design new CTC circuits from these circuit modules.
Bear in mind that CTC signaling systems are not simple systems. They have evolved over decades to provide the railroads with the train control and safety they need in their day-to-day operations. They are complex and costly, but they are also very effective in moving the most tonnage over the existing trackage safely. There are many different flavors of CTC systems, with each railroad adapting the general system to their specific needs.
The CTC system we have described here is overkill for many, or even most, model layouts. In these cases the system we described in the April 1994 Mainline Modeler is simple, easy, and provides basic CTC functionality. But for those who want a complete prototype CTC system for their layouts, the system we have described here is the simplest, most modular, and adaptable system we have yet seen. The operation of this system has been checked against that of the prototype and, while there are many flavors of prototype CTC, this system corresponds to a common type. The circuit modules we have described can be used to implement other flavors of CTC as well, using the principles we have outlined here.
This description of our CTC system is the last of the signaling articles we will present. Next time we will discuss a very high-performance walkaround memory throttle, featuring outstanding low speed performance and safe cool operation of all types of DC motors. Even the most demanding of model engineers will find this to be among the best throttles in the hobby. We'll see you then.
While we carefully go over each of these articles several times looking for errors, we do make mistakes. In the May 1994 Mainline Modeler, we discussed using the SeeTrain optical detector circuit to control a bi-color LED on the panel. The part number we gave was Radio Shack 276-025, which is a three-legged bi-color LED. The correct part number for the two-legged bi-color LED required is Radio Shack 276-012. Many thanks to our alert readers for spotting the error.
