>Not sure what you're trying to prove. I never asked the question in the context of rules compliance.

There are others who read these threads who are interested in the technical details.

>I can't think of any scenario (other than curiosity) why an operating employee -- on a moving train, travelling in the direction authorized -- would need to be concerned with the opposing signals being displayed behind their train.

Agreed, but in case you hadn't noticed, this has generated a lot of speculation as to why the signal "should have been" green, yellow, or red. So I was trying to provide some technical details. All to often speculation becomes the basis for urban legends, which in turn become "facts". Many readers here are genuinely interested in facts, not legends.

When all is said and done, there's no way to make a determination today as to what the signal(s) should have displayed then.

> As I've posted elsewhere in this thread, there
> isn't sufficient information available today as to
> what characteristic of the signal system's
> circuitry may have kept those signals you saw in
> the Approach ("yellow") position.

>Did you see where I explained that my SP friends, who had worked south out of SLO for many years by that point, said it was normal behavior for those ABS signals? They had seen the same thing thousands of times...

I'm not disputing what they saw because I've seen similar behavior. What I'm disputing is the resulting generalized conclusions on how single track ABS/AP-B signals "should" work. It's not much different than saying, "I've seen the sun travel across the sky thousands of times, so therefore the earth can't be moving".

>UP finally upgraded the signals a few years ago, but the old design had lots of staggered signals and inconsistent block lengths. Lots of short 1-mile blocks.

There never has been a requirement that blocks should be a certain, fixed length. The SP more or less placed their signals according to the stopping distance of the heaviest trains coming to a halt from their maximum authorized speed using a full service reduction. In other words, go by a yellow, make a normal stop (immediately) and you will stop before going by the red.

Most RRs tended to avoid staggered signals unless operating considerations dictated their use because in staggering signals an addition another set of track relays and signal control relays is required. Last time I checked, track and neutral line relays cost somewhere in the neighborhood of $800-1,000 each. When you adjust for inflation, in those days, the costs were comparable, and each signal typically required about three or four of these relays.

Since about 1990, the RRs have adopted a 2 mile block (some times longer) as standard, regardless of track speed and train length/weight. I have no idea why, other than this is the flagging distance, and it certainly has slowed operations, particularly when there need to be close headways.

> Some of the sidings had (maybe still have?) non-controlled absolute signals.

"Non-controlled absolute signal" is a new term which dates from the mid 1990s (ISTR it's a concept the FRA came up with). UP has put so many of them all over, even without distant signals ahead of them, that they've become traps without much meaning. If we are discussing what the SP did 20-40+ years ago, the term didn't exist.

The only sidings which would have "non-controlled absolute signals" would be those with spring switches and exit signals in the siding. The signals on the main line opposite these exit signals might or might not have number plates.

The only time I post comments occurs when speculation about what railroads actually did gets too far afield and wildly ungrounded. If you're not interested in what really happened, how the rules are applied, how signal systems were developed and worked, feel free not to read my posts.