Thanks for the confirmation. And your analogy caused the light bulb to go off.
I had assumed that the loco‘s body/frame meant that the front and rear wheels were electrically connected (as you aptly describe), but I was not sure why that would matter.
I was thinking that the AR-wired segment from which the train had departed (AR-wired segment 1) would have electrically “aligned“ with/matched “polarity” with that of the non-reversing segment as the loco crossed the gap and the AR circuit did its magic. Then the same thing would happen as the front wheels of the loco passed over the gap to the next AR-switched segment (AR-switched segment 2). I assumed that would simply cause AR-switched segment 2 to align with the non-switched segment and, by extension, with AR-switched segment 1 (which was already aligned with the non-switched segment).
What I didn’t consider, however, was the potential for the electrical continuity of the loco (and, as you point out, any electrically connected cars, such as a wired tender) to transmit the electronic message of a “short” experienced by the loco’s front wheels crossing the second gap (between the non-switched segment and AR-switched segment 2) to the AR circuit controlling (protecting) AR-switched segment 1. That AR circuit would either not know what to make of the confusing message or, more likely, would try to switch the “polarity“/alignment of AR-switched segment 1 “back” (after all, that’s what AR circuits DO). We’re talking very short time intervals here. Such that the switching action of the first (rear-most) AR circuit would have “rebounded” back to the second (forward-most) AR circuit, and a “war” between AR circuits initiated.
In effect, by spanning the length of the non-switched segment, the electrically connected elements of the loco cause the two AR-wired segments to no longer be electrically separated by a non-switched segment!
Ergo, the minimum length requirement for the non-switched segment.
Now that I’ve “got it“ (thanks Blair), I hope others do too, as I don’t believe the rationale for the rule (as opposed to the rule itself) had been made clear by the previous discussion on this forum.
N&W Steam Only