This is the case in full-sized practice. If a train doesn't have authority to occupy a track it cannot be moved, even if the nearest facing signal is CLEAR. APB prevents conflicts that arise when train movements are not to schedule, especially when opposing movements are involved.ChuckHackett-844 wrote:When you get into bi-directional running, I assert that ABS/APB signals are not sufficient without requiring someone to back up in the case of APB conflicts (e.g.: meets at intermediate signals - not desirable at a large meet). Further, managing traffic flow and preventing deadlocks requires using a) a dispatcher, b) timetables, c) track warrants, etc. (which is not practical for 90% of ride-on railroads).
APB in riding scale is doable, but will in practice be unworkable unless a club is very serious about formal dispatching and enforcing operating rules. A big part of the problem is most riding scale hobbyists may know about railroads and trains, but little about railroad operation and practice. This can range from simple things like failing to close a turnout after setting out a car, to serious issues, such as not understand and/or obeying signals. The latter, in particular, illustrates something that another Chaski user has frequently noted: a signal system is only as good as those who operate on the signaled track. If train operators can't be relied upon to obey signals then riding scale APB doesn't have a ghost of a chance of being successful.
For the benefit of readers who may not be familiar with railroad signaling, above is an illustration of a stretch of APB territory track, with no trains present. Odd-number signals face west-bound (WB) trains and even-numbered signals face east-bound (EB) trains. Both sets of signals are affected by movements in either direction. With no trains in the area, all signals will be displaying CLEAR aspects.
Signals 1, 42 and 51 are referred to as "headblock signals" and are "absolute," which means these signals cannot be passed while displaying a STOP aspect, unless authority to violate the signal is granted by dispatch. All other signals are permissive, which engineers may treat as "stop and proceed." In this case, "proceed" means to run at restricted speed (10-15 MPH or thereabouts) and expect to encounter another train, or a track defect that caused the signal to display to STOP. Incidentally, the acronym APB (absolute permissive block) comes from this mix of absolute and permissive signals.
Note the presence of passing tracks, which allow meeting trains to pass or overtake. Despite the positions of signals 2, 41 and 52 relative to the passing track entrances, once a train is completely in the passing track it is no longer under signal protection. Operating rules dictate how trains are to be moved once off the mainline.
In the next illustration, a WB train has passed signal 1, at which point it has entered the west-bound headblock. Block occupancy detection is bi-directional so the APB can tell which way a train is going. This feature is signified by the "cut" through the track midway between signals.
Now the opposing signals 12, 22, 32 and 42 have all "tumbled" to STOP. So far, so good. However, here is where the deadlock Chuck mentioned can develop.
Suppose that right after the WB train passed signal 1 an EB train passes signal 52 and its engineer having misunderstood his train orders, fails to take the passing track. The EB engineer will see signal 42, located at the east end of the passing track on which his train is supposed to be, change to STOP. However, he's highballing with 100 loaded coal hoppers and can't stop before reaching the signal. This being the case, he will pass signal 42 and occupy the east-bound headblock, causing opposing signals 11, 21 and 31 will tumble to STOP. The resulting signals status would be as illustrated below.
Needless to say, both trains must immediately be stopped. Assuming the engineers are alert and promptly react when they see their facing signals change to STOP, the EB train will stop before reaching signal 32 and the WB train will stop before reaching signal 21, this being the worst-case scenario. Although their respective signals are permissive and hence can be passed if safe to do so, the two engineers will see each other at this point and will know to not proceed. The line is deadlocked and dispatch would have to determine which train is superior and instruct the other train's engineer to back down and take the passing track. In practice, this seldom happens on full-sized railroads, as they don't operate with anywhere near the train density seen at some Live Steam clubs. However, should it occur, the risk of a cornfield meet will not exist, as long as both engineers follow the rules and obey the signals. Of course, should one train experience a brake problem all bets would be off.
I can't answer that without doing a full analysis of all possible movements. As time permits, I will see what I can conjure.How (as an example) would one signal a Wye using only ABS/APB signals to support bi-directional running? (for complete description of the requirements see my initial post in this thread and the clarifications that followed - the diagram, etc.)