Modeling C&O CTC--part 2: Clearing an interlocking signal

Part 1 of this ongoing series appeared in the January/February 2002 issue, and gave some background on Centralized Traffic Control (CTC) and its applications.

Let's now look at the requirements for clearing a signal for any given route through an interlocking. I will list each condition that the signal system will check-this should help in setting up your own model signals to operate more like the prototype. Looking at Figure 1 (on page 6), let's assume that there is a westbound train approaching signal L88 at the east end of the siding. The dispatcher has decided that this siding will be the best place for the westbound to meet an eastbound. The dispatcher, from his console, will send out the control code to reverse switch 87. Before switch 87 will throw, the signal system will check three conditions:

1. That no train is within the interlocking limits (occupying the detector track circuit) this protects against throwing the switch under a train.

2. That no home signals at this interlocking are cleared for a route over the switch.

3. Against throwing for a predetermined time delay, after a home signal governing movement over the switch is cleared, and then put back to stop by the dispatcher.

Condition 3 is known as "time locking." Time locking protects against the dispatcher clearing a signal for an approaching train, then putting the signal back to stop and changing the route. When the dispatcher manually puts the signal to stop, the signal system will make him wait a set time before he can change the route or clear the opposing home signals. This time interval is to allow the approaching train sufficient time to stop safely before its original route is changed. The formula that we use on the railroad to determine this time interval takes into account track speed, curvature, and other pertinent train-handling factors. It can be as short as two minutes and as long as eight minutes.

Given that many modelers run their railroads using a "fast clock," running eight or even two minutes of time locking would not be practical. My experience has been that a delay of one minute is sufficient and seems to work quite nicely. You may find that you prefer a longer or shorter time interval for your railroad, or you may not want to emulate time locking at all. If any of the above three requirements are not met, the switch will be locked and cannot be thrown under power.

After switch 87 has thrown to the reverse position it will indicate to the control machine that it has responded and is now in the reverse position. Next the dispatcher will send out the control code to clear signal L88 for the westbound train to enter the siding. Once again, the signal system will check several conditions.

1. That no train is occupying the detector track circuit or the block protecting the siding. This block extends between signal LC82 and RC88.

2.That home signal RC88 is not cleared for an eastward movement and that this signal is not in the process of running time for time locking.

3. That no train is occupying the detector track circuit at the west end of the siding and that signal R82 is not cleared or running time. Note that this condition is only checked if switch 81 is in the reverse position.

4. Switch 87 is in the reverse position.

If all these conditions are met, signal L88 will display a red-over-yellow aspect and the field equipment will indicate to the dispatcher that this signal is now cleared. The red-over-yellow aspect is a restricting indication (Rule 290). This signal informs the engineer that he may proceed, not exceeding 15 miles per hour, and be prepared to stop short of an obstruction or anything else that may require him to stop. Aspects and indications will be discussed more thoroughly in Part 4 of this series.

Now the dispatcher must clear the appropriate signal for the eastbound train. Assuming switch 81 at the west end of the siding is in the normal position, the dispatcher will send out the control code to clear signal R82. The signal system will check the same conditions that were previously checked to clear signal L88, but for the route signal R82 will be governing down the main track.

1. That no train is occupying the detector track circuit at this interlocking and the block between signal L82 and R88 is not occupied.

2. That signal L82 is not cleared for a westbound movement or in the process of running time.

3. Switch 81 is in the normal position.

Considering that switch 87, at the east end of the siding, is already in the reverse position to allow the westbound to enter the siding, the signal system-knowing thiswill not check signal L88 or the detector track circuit at the east end. Once these conditions have been checked signal R82 will display yellow over red. This yellow-over-red aspect is an approach indication (Rule 285) and informs the engineer that he may proceed, not exceeding 30 miles per hour, and be prepared to stop at the next signal (i.e. signal R88). The dispatcher has now successfully set up a meet between two trains. Ali of this took him less than one minute to do. This is what makes CTC so efficient. The dispatcher has no train orders to write and relay to a telegraph operator, who then has to "hoop them up" to the appropriate train. Each train would then have to stop and throw the appropriate switches by hand.

After the westbound train has passed the "cleared" signal (i.e. signal L88), it moves into the interlocking limits and occupies the detector track circuit. The signal now automatically returns to "stop," and the field equipment will indicate to the control machine that the detector track circuit is occupied and that the home signal is at "stop."

Once the last car has cleared the interlocking limits and is no longer occupying the detector track circuit, the dispatcher will send out the control code for switch 87 to throw to the normal position. After the switch has thrown to the normal position and has indicated to the control machine that it has responded, the dispatcher will clear signal R88 for the eastbound train to proceed. The signal system will check several conditions to insure that the track conditions are safe for the eastbound to proceed:

1. That no train is occupying the detector track circuit or the block that extends between signal L88 and the next governing signal.

2. That signal L88 is not in the process of running time.

3. That switch 87 is in normal position.

4. That all westbound intermediate signals between the east end of this siding and the west end of the next siding are in the red position and that no westbound signals are cleared or running time at the west end of the next siding.

Part 3 of this series will look at intermediate signals and the conditions required to clear a signal into single track between sidings. If all four of the above conditions are met, signal R88 will display green over red. This green-over-red aspect is a clear indication (Rule 281) and informs the engineer that he may proceed at authorized track speed.

Whether you are modeling the singletrack line with passing sidings of the Mountain Sub or the massive interlocking at RU Cabin in Russell, the general principle is the same. The signal system will always make the appropriate checks to see if the route requested by the dispatcher and/or the operator is indeed safe for train movement.

Copyright Chesapeake and Ohio Historical Society, Inc. Jun 2002
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