AutoReverse for multiple segments


I believe I know the answer:  NO; it can't/shouldn't be done.  But I just wanted to confirm that it's a bad idea to attempt to wire two separate reversing segments through a single AR switching device.

Question:  Where there are two physically and electrically separated reversing segments, both ends of each of which are physically adjacent to (but electrically isolated from) non-reversing segments of commonly-wire track, can the reversing function for both reversing segments be performed by a single AR switch, or must/should each reversing segment have its own reversing switch?  I haven't seen this issue addressed here or elsewhere.  Is the answer the same, even if the non-reversing segments (and therefore each of the adjacent reversing segments) are in separate power districts?  [I don't see why the answer is not the same.]

I recognize that one reason that separate reversing switches could be required is the possibility that two trains might enter into the different reversing segments one after the other, and thereby occupy both segments simultaneously.  All would be fine with the first train, where the AR switch would function normally, and simultaneous switching the polarity/phase of both reversing segments would have no consequence for the unoccupied other segment.  But should a second train enter the other reversing segment while the first train is still in the first segment, the second train would also "trip" the AR switch, which would again switch the polarity/phase of both reversing segments.  That would undo the polarity/phase compatibility previously achieved for the first train, resulting in the AR switch changing "back" the polarity/phase of the first segment.  Of course that would simultaneously change the polarity/phase of the second segment, resulting in a new incompatibility for the second train.  Which, in turn, would result in the same AR switch changing the polarity/phase of second segment (as well as that of the first segment).  And the process would repeat itself, back and forth, in an accelerating and fruitless effort to attain impossible-to-achieve simultaneous polarity/phase compatibility of the the two reversing segments -- until the switch burns itself out.  

Saying one will avoid running two separate trains in the described manner is hardly a solution.  As we all know, if something can go wrong it will go wrong.  So while I suppose it is theoretically possible to wire separate reversing segments to a single reversing switch (I imagine configurations could be posited in which it might work), best practice would certainly not approve.  

But, apart from the obvious problems identified in the above operational scenario, is there a more fundamental reason why, as a matter of electrical engineering principles, two reversing segments should not be wired through a single AR switching device? 
Bill D
N&W Steam Only

Don Weigt

Bill, As long as the distances aren't astronomical, and the currents aren't extreme, I can't see any engineering principles involved here. Each reverse loop or wye having its own AR is strictly for practical reasons: because two trains might be at different thresholds simultaneously, setting up an impossible requirement for both phases simultaneously from a single output.

I actually violated that rule on my own railroad. I have a double track reverse loop. Immediately adjacent to each end, turnouts join the two reverse loops to single track main line segments. The boundaries to the reversing loops are at the diverging ends of the turnouts. Each of those reversing tracks is about 27 feet long, able to hold three of my typical length HO gauge trains. Connecting them to one AR menas I can't have a train exiting westbound from either track while another is entering eastbound at the other end. I accepted that limitation, since I usually operate the railroad by myself, and never felt the need to have trains entering and leaving those tracks at the same time.

BTW: the same requirement exists for each reversing segment, be it a loop or wye. Trains can't cross both boundaries at the same time, at least not powered units, or lighted or other cars with multiple metal wheels connecting to the power from each rail. If trains connect the rails across both boundaries at the same time, a short is inevitable. Usually, it's enough to have to observe the rule for one reversing loop or wye, without adding the needs of another to the equation.

So, the core reason is, having separate ARs for each reversing section prevents a lot of complex restrictions when running multiple trains.

Don Weigt

Don Weigt


Thanks Don. The operational limitations I understand. And obviously a number of practical considerations will affect individual decisions. But I am gratified to learn that there isn’t some engineering principle at work of which I was unaware.

Still learning, but much appreciate the knowledge I have gained through this blog.

Bill D
N&W Steam Only