Having discussed the various systems, there are other things that are used in combination with the authority systems. Since they overlap many of the previous subjects, I thought I would just hold them to the end and cover them with the other systems as a backdrop.
Automatic and Manual block systems
Even though these are very different systems in what makes them work, how the signals are set up and the eras in which they were commonly used, their relationship to the other systems are similar.
In a manual block system the block stations or towers communicate with the station ahead or behind a movement to indicate whether a move is coming or has passed. The train stops or lets trains pass based on whether the block ahead of them is occupied or empty.
For this discussion ABS is considered the same as APB.
Block signal systems provide protection for train against following and opposing trains. They indicate the condition of the block ahead of a train. Manual block only indicates the occupancy of a block by a train, automatic block signals (ABS) indicates train occupancy, switch position and rail continuity. Both manual and automatic block signals do NOT supersede the superiority of trains and require TT&TO or TWC (or modern equivalent) over the same territory to provide the main track authority. Manual block signals are at or near manned block stations. ABS signals are spaced at semi-regular intervals along the railroad, but are not configured the same way as CTC signals.
At a controlled siding switch in CTC (a control point), there will be a signal at each of the three entrances to the switch, with all the signals facing away from the switch. Typically, the signal beyond the siding will have two heads to indicate a diverging route or a reduced speed through the turnout. In ABS there are two signals beyond the points of the switch, one in each direction, and neither of them display diverging (or equivalent) aspects. The switch is manually controlled.
Since ABS provides protection against following moves, the rules may allow following movements in the same limits or block in TWC or DTC. ABS might also extend through yard limits.
ABS on a model railroad has less utility than CTC, it doesn’t provide as much information on switch position and basically keeps trains from running into each other which competent operators should be able to accomplish themselves.
Yard limits
Yard limits is a method or authorizing movements on the main track. They do not apply to a yard or tracks other than a main track, just the main track. They don’t really have anything to do with how far a switch engine can go (that’s “switching limits”). In yard limits, higher class trains may proceed at main track speed, all other trains and engines may use the main track, not protecting against other trains and engines, except ty have to clear the times of higher class trains. They proceed at some reduced speed, prepared stop short of or within half the range of vision of, obstructions. Higher class trains are first class trains and in some rule books, 2nd class trains. If you read a rule book a yardmaster has nothing to do with yard limits, they aren’t even mentioned. There is nothing in the rules that require a train to stop and get permission from a yardmaster to enter yard limits. In rules associated with or derived from the PRR, the dispatcher or control operate may authorize trains to enter yard limits.
Since yard limits is way to authorize movement on a main track, you can have yard limits without there being a yard there or a yard without there being yard limits. On some branches they operate the entire branch under yard limits. Yard limits do not eliminate the superiority of trains, trains schedules still exist and first class schedules have to be respected. Train orders also apply in yard limits. ABS, TWC, and DTC alI can overlap yard limits. In more modern times there have been efforts to not overlap TWC and yard limits and generally its not good to overlap yard limits and CTC (it doesn’t improve anything, a trains still need permission to enter CTC).
In modern rule books yard limits is pretty similar, except the requirement to clear the schedule of higher class trains is gone, most require them to not delay passenger trains and to minimize delay to other trains.
Yard limits are a handy tool on a model railroad allowing trains and engines to enter the main track without something from the dispatcher. The risk is ending up with too much of the layout in yard limits.
Track other than a main track
This technically is outside of the scope of the other subjects, since it, by definition, doesn’t involve the main track, but I’ll throw it in here because its actually what authorizes movement on many industrial switching layouts. Since the track it covers is not a main track, the superiority of trains doesn’t apply. Track on industrial leads or in yards or industrial areas is generally track other than a main track. Rule 105 in most older rule books covers that. Trains and engines must move basically the same as in yard limits except that they don’t have to clear any other trains and may be restricted to a slower speed.
PTC
Positive Train Control (PTC) is not really a main track authority system, but it is an overlay. Mandated by Congress on lines that carry passenger trains and meet certain thresholds of hazmat, PTC is a safety overlay on top of the existing systems. Older safety systems (cab signals, automatic train stop, overspeed systems, etc.) stop or restrict a train if it passes a restrictive signal too fast or without stopping. They restrict a train after it fails. PTC is designed to be preventative. It takes into consideration the location of the train, the limits of the authority, positions and locations of switches, the speed of the train, the weight of the train and the acceleration/deceleration of the train calculates whether it’s possible for the train to get stopped before it exceeds it’s the limits of its authority or is exceeding the authorized speed. If the train could get stopped and isn’t speeding, PTC does nothing. If the train might not get stopped before exceeding its authority, or is exceeding its authorized speed, it stops the train.
In a perfect world, if all the systems work right and nothing breaks, if the crew is obeying the rules they should never know PTC is there (don’t want any comments from train crews about how many times it fails, I said IF it was perfect). PTC does not run the train. PTC has nothing to do with making the train go, its all about making the train stop. The concept is to prevent catastrophic, mostly human caused accidents.
Hopefully this series has provided you with a general overview of main track authorities and given you some ideas on how to model them.