>Mark's observations seem valid to me.

The point I was trying to make, albeit a bit indirectly, is that even by using binoculars, which is specifically forbidden on some RRs, there is no way to tell for certain whether or not you are "being followed" by another train, and it's the "for certain" which keeps you employed (and alive).

>This was SP's pre CTC setup for ABS teritory. Signals would clear at the intermeiate blocks behind (going away) the train.

Normally this was the case - with all SP ABS. Sometimes CTC intermediates do this, other times not because it depends on how the directionality control of the intermediates has been arranged ("not" can be considered general practice on SP's CTC).

>Signals at the end of siding would behave differently,

Not necessarily. If you could see the signal indications with no trains around, which was difficult because the SP used approach lighting, you would find that all of them would be "green".

>more based on approch circuits.

Except for a few overlaps, the only "approach circuits" I ever saw were in sidings where the exit point was a spring switch.

>Take siding lights would apply.

On the SP there were very few if any take siding lights on ABS Single Track because by definition the crews were supposed to figure out what to do on their own initiative and act accordingly.

In all my travels on the SP, I've seen not more than about three: two for for Castroville on the "poor man's" CTC between Salinas and Watsonville Jct, which isn't ABS Single Track and which are still controlled directly by the DS, and one was on the ABS DT for the West siding at Davis, which was controlled by the operator at Davis rather than the DS.

>Sidings had spring switches to save time leaving.

This depended on the RR. The AT&SF on the Valley had them on almost all sidings; on the Coast the SP had them on only a few sidings; on the West Valley, none.

Generally speaking, this: www.lundsten.dk/us_signaling/abs_st_sp/p_index.html describes the operation of SP's Single Track ABS setup, although it gives no details about their wiring.

However, it errs slightly with signal locations. For the most part the intermediates were opposite each other, not staggered as shown. In addition, all signals at the ends of sidings are shown to be in advance of the switches in the direction of travel. On the SP this was true only for spring switches. Where there were no spring switches, the signals at the ends of the siding were directly opposite each other on either side of the track, with the entire siding, switches and all, being between the signals at each end of the siding. And even this wasn't a blanket rule because the siding at Perry, between Coyote and Gilroy, had spring switches at each end, yet the signals were located as though there were no spring switches.

The non-spring switch signal locations is the way GRS first designed the system.

>Other roads seem to prefer a directional 'stick circuit' were only intermediates in one direction between siding would clear. The visible signal behind your train would stay red.

Not necessarily because all RRs pretty much adhered to the principles shown by AAR Standard Plan 8045A (for seachlight signals, track signaled for movement in both directions). When the stick relay picked up, it would permit following movements. For opposing movements, since it didn't pick up, the signals would "tumble-down" to red all the way to the next siding. The opposing signal behind your train would display an indication appropriate to the block occupancy beyond it.

All of what we're discussing must be considered only general in nature because without access to the written circuits, it's impossible to determine with 100% certitude what would happen with any given signal because of what I mentioned earlier about the removal of smaller sidings and non-rewiring of the overall ABS system. This is one of the reasons why even though the basic circuitry and principles were GRS's AP-B, the SP didn't define it as such.