>SP used to have double track with ABS over much of Donner Pass, but between Truckee and Norden it was operated as two main tracks.

This was an early stage of the plant. By the mid 1960s, the interlocking ran between Switch 9 and Truckee, and it was defined as Interlocking, not "two main tracks" (which is specifically a CTC-ism).

>Other than transferring who and where the control operator was, I don't know of any actual changes between Manual Interlocking and CTC.

This is correct, other than the rules which applied (which were essentially identical anyway).

>Absolute Permissive Block (APB) is another interesting one.

See the paragraph in the upper right of page 15 in this: www.alstomsignalingsolutions.com/Data/Documents/History.pdf

"AP-B" stands for "Absolute Permissive Block". It was developed by GRS in the teens, and GRS Bulletin 135 of 3/1919 describes the system in detail. The name was chosen because it provides Absolute protection between sidings, meaning no trains traveling towards each other, but Permissive indications between sidings for following movements, meaning for following movements two trains could be between sidings, using Block signals to convey Main Track authority. AP-B used a stick relay at each signal to detect the direction of train traffic, and the operation of this series of stick relays between sidings is commonly referred to as the "tumble-down".

One thing to keep in mind is that while most single track signal installations were wired like AP-B, they weren't used as such unless the company said so in the timetable.

By the 1960s, on the SP, there was virtually no "textbook" nor TT-defined AP-B in California, because the SP had taken out so many short sidings and not rewired the signal system to run the tumble-down around these former siding locations.

>There is an issue if two opposing trains are approaching each other with two unoccupied signal blocks between them, which would result in each train getting a clear signal.

Ahh, no because this is taken care of by a directional stick relay, the tumble-down. Refer to AAR standard plans 8001 through 8009, 8027, 8029, and 8079.

>Consider the typical SP signals that used lower quadrant semaphores on single track. The original design only used a single arm that offered either a clear or stop aspect with signals in flat pairs. Unless a train is operating at restricted speed, it must be given some kind of approach warning that it may need to stop at the next signal. This type of ABS used "Permissive Block". When a train passed the last signal at a siding location, all the opposing signals to the next siding switch would "tumble down" dropping to red.

OK, while true, this somewhat contradicts your preceding paragraph.

Lower quadrant semaphores can give only two indications: clear and stop. In order to have an approach indication, which gives an advance warning of a home signal in stop position, a distant signal is required. On the SP, the home signals were those red blades with the square ends, while the distant signals had yellow blades with fishtail-cut ends. Lower quadrant home signals could give only two indications: clear and stop. The corresponding distant signals also had two indications: clear and approach.

For night indications, in the early days (all RRs), clear was a white light, approach was green, and stop was red. Starting in the teens this was gradually changed to today's green, yellow, and red. This is also the origin of the phrase "getting color" which you might hear from time to time on the radio..

>Was CTC trademarked by US&S?

No, GRS

>There used to be abandoned concrete signal footings that indicated the original signal arrangement had been staggered pairs. Was this originally ABS?

Yes.

>If there was a failure resulting in a track light, the dispatcher could not reverse the direction of traffic

This depended on what was causing the TKO (TracK Occupancy light) and where it occurred, because it interrupted the tumbledown between controlled signals. Frequently the directionality of the signals remained "set" for the direction of last use or would tumble only to the point of TKO cause.

>I'm not sure if all CTC systems used a direction of traffic or permissive block arrangement.

CTC intermediates also use the tumble-down.

>Since the dispatcher knows if the track between absolutes is occupied or not,

If there's a TKO between sidings, all the DS can do is ASSUME it's an occupied block of some kind because that's all track circuits do: indicate whether they're clear of a train or occupied. Broken rails etc. are automatically interpreted as the block being occupied.

>I know one of the early arguments in favor of PTC is that railroads would save so much money because the only wayside signals required would be at control points.

While PTC is supposed to give the dog a bath and put out the cat each night (and slice your bread for you), it won't replace track circuits until is has the same fail-safe broken rail protection capability.

>What do you call the setup used in some transit systems (especially light rail) where there's a central control but not really a dispatcher?

These are not railroad opeartions, so the Statues, Regulations, Rules, and operating practises are different.