Centralized Traffic Control (CTC)

CTC refers to a method in which a dispatcher controls the routing of trains on a railroad. The dispatcher sits in front of a large panel of levers, pushbuttons, and indicators. He or she is able to "see" which blocks are occupied and can therefore keep track of the trains in their territory. This is similar to the air traffic controller sitting in front of a radar screen. The dispatcher also has control over trackside signals typically governing train movements between single track and double track territory (e.g. a passing siding). However, intermediate signals along the single and double track territories are not typically controlled by the dispatcher. These signals usually operate in either ABS or APB mode. In all cases the signal interpretations on many railroads adhere to The General Code of Operating Rules.

The figure below is representative of a section on a dispatcher’s panel. You will see from the track "schematic" that there is a double track section converging into a single track. The circles in the center of the tracks indicates block occupancy. We’ll use Red to indicate an occupied block and Black to indicate an unoccupied block. The turnout is a mini-block in and of itself. This is often called an OS section; OS has different meanings to different railroaders, but a common meaning is "Over Switch." The OS section has its own block occupancy indicator. Also on the track diagram are symbols representing the trackside signals surrounding the turnout; these signals protect the OS section and the corresponding blocks connected to it. Some prototype dispatcher panels had working indicators here. These were essentially repeaters of the associated trackside signals. Below the track diagram you’ll notice that there is a lever numbered 11 with labels "N", "R" and "SWITCH" on it. This is the turnout control lever which has just two positions as outlined. Above the turnout control lever are lights which indicate the position of the turnout. The other lever, numbered 12, has labels "L", "R", and "SIGNAL" on it. This is the signal control lever. It actually has three positions: left, stop (center), and right. It controls the direction of traffic flow through the OS section. When in the "L" position traffic is cleared from right to left (i.e. leftbound). In the "R" position traffic is cleared from left to right (i.e. rightbound). In the center position no traffic is cleared and signals associated with the OS section will be red. Above the signal control lever are lights which indicate the state of the trackside signals associated with the OS section. Below the signal control lever is a pushbutton label "Code." This is a very important part of the CTC panel operation. The actions caused by the position of the turnout and signal control levers do not take effect until the code button is pushed. Therefore the purpose of the indicators above the levers is to show the current conditions of the turnout and signals. The position of the levers shows the conditions that will result when the code button is pressed. There is an important exception to this last statement. If the OS section is occupied then pressing the code button will have NO effect. This is a protection mechanism so that the turnout is not thrown while a train is over it! In addition, the dispatcher should not be allowed to change the signals associated with the OS section if it’s occupied.

There have been several sources of information on CTC in the model railroad press over the years. Some of them are listed below.
bullet How to OPERATE your model railroad, by Bruce A. Chubb, Kalmbach Publishing Co., ISBN 0-89024-528-2
bullet "Centralized traffic control for the Sunset Valley", by Bruce Chubb, Model Railroader, January 1984, pages 186-193.
bullet "Centralized Traffic Control for the Cat Mountain Line", by Ken Thompson, Model Railroader, May 1984, pages 74-80.
You will notice in the following CTC diagrams that the signals on either side of the turnout have the same number. This is how most railroads label their dispatcher’s panels. In fact, turnouts are usually assigned odd numbers and signals get even numbers. This may seem contradictory to the labeling we used in the ABS and APB diagrams. Don’t get hung up on this! You can label your signals any way that suits you best. Now, on to the CTC panel ...  
Notice in the above figure that the turnout control lever is in the "N" position and the signal control lever is in the center position. Since there are no trains occupying the OS section when the code button is pressed the turnout will be aligned for the through (or straight) route and signals 12L & 12R will show red indicating that no route is cleared. Notice that there are no mileage markers on these signals which indicates that they are absolute. Typically, all signals associated with an OS section are absolute. Recall that the code button must be pressed for these actions to take effect. We’ve chosen a green light to indicate the turnout is aligned straight and yellow to indicate when it’s aligned for the diverging route. Some railroads used yellow (or white) and red instead. Choose the one you like best!

In the figure below the signal control lever has been moved to the "R" position in order to clear the now present rightbound train into the single track territory. Lower signal 12R has been cleared and will probably show Green but could instead be Yellow or Flashing Yellow depending upon block conditions to the right of the OS section. Upper signal 12R will show Red since the turnout is not aligned for its route. Signal 12L will show Red over Red since leftbound traffic is not cleared.

At this point the rightbound train is proceeding along the single track territory. Assume the dispatcher has a leftbound train waiting in the next double track section to the right. The dispatcher can now try to set up switch 11 and signal 12L for that train. The figure below shows that the dispatcher has moved the switch control lever to the "L" position and the turnout control lever to the "R" position indicating that the leftbound train should take the diverging route. Notice however that the previous rightbound train is still occupying the OS section. As such the new control lever settings cannot take effect. You can see that the indicators still show the previous turnout alignment but notice that the red indicator above the signal control lever is now illuminated. Once the OS section became occupied the clear indication got "knocked down."

Once the rightbound train has cleared the OS section the dispatcher can retry the new setup. As seen below the leftbound train has been cleared to the diverging route. Signal 12L will show Red over Green since turnout 11 is in the "R" position. The indication could instead be Red over Yellow or Red over Flashing Yellow depending upon block conditions to the left of the OS section. Signals 12R will both show Red indicating absolute stops for rightbound traffic.

We carry CTC switches and plates to help you implement a CTC dispatcher’s panel as illustrated here. The related items are:
bullet #R628-61   CTC Panel Knob & Plate Kit which includes all necessary plates, levers and knobs for a complete OS section
bullet #RS2POS   2 position 6 pole rotary switch for use as a turnout control lever
bullet #RS3POS   3 position 4 pole rotary switch for use as a signal control lever
bullet #PBSPST   momentary ON pushbutton switch for use as a code button

For the various indicators we recommend LEDs available from Radio Shack and other electronic component outlets. There are basically two popular sizes of LEDs. One is known as a T-1¾ (5mm diameter) while the other is known as a T-1 (3mm diameter). We recommend the T-1¾ size for the turnout control and signal control indicators. You could use either size for the block occupancy indicators on the track diagram. Snap-in holders can be used to "dress-up" the indicators. The following Radio Shack parts are listed as suggestions.

bullet #276-022   Green T-1¾ LED (pkg of 2)
bullet #276-021   Yellow T-1¾ LED (pkg of 2)
bullet #276-041   Red T-1¾ LED (pkg of 2)
bullet #276-079   Snap-in holder for T-1¾ LEDs (pkg of 5)
bullet #276-026   Red T-1 LED (pkg of 2)