Recent discussions have brought to light how many variations there is in signal technology and terminology. Consider the following:

Interlocking is defined as "Interlocking Signal appliances that are interconnected so that
each of their movements follows the other in a proper sequence. Interlockings may be operated manually or automatically." Early manual interlockings were the "Armstrong" variety that required the operator to move large levers in sequence that controlled signals and threw switches through a system of connected rods (usually heavy pipe). The interlocking machine had a system of locking bars, levers and locks that locked or unlocked the levers. This evolved to interlocking machines still using a mechanical locking bed with "pistol grip" handles while changing the signals and switches pneumatically or electrically. This again evolved to relay logic, and is now done by software. The control operator was originally in an elevated tower overlooking the interlocking plant, but now might be thousands of miles away. A track circuits became reliable, automatic interlockings became common at many simple crossings. Automatic interlockings are typically triggered by approach circuits and clear the route for the first train to arrive.

Automatic Block Signal System (ABS) is defined as "A series of consecutive blocks governed by block signals, cab signals, or both. The signals are activated by a train or by certain conditions that affect the block use." In ABS, the signals typically default to green or clear. The primary purpose of ABS was to provide protection against following trains, though depending on arrangement it can offer protection against opposing movements. An important thing to remember is that ABS by itself does NOT provide authority to occupy the main track, but it acts as an overlay to TT&TO, TWC, DTC, YL, RL, Double Track, etc.

Centralized Traffic Control (CTC) is now defined as "A block system that uses block signal indications to authorize train movements." The basic principle of CTC is to have Control Points (The location of absolute signals controlled by a control operator) typically at siding switches, junctions or crossovers where the switches are powered. These control points use the same operating principles as a manual interlocking, while any intermediate signals operate similar to an ABS system.

Note that in early manual interlocking without track circuits, it was the control operator's responsibility to determine track occupancy. With modern manual interlocking and CTC control points, the control operator does not directly control switches or signals but requests their change. Local circuits determine if and when it is safe to change a switch position, unlock hand throw switches and the resulting signal aspects.

Controlled Sidings are defined as "A siding within CTC or interlocking limits where a signal indication authorizes the siding’s use." Controlled Sidings are not a main track be definition, but most of the same rules apply. On uncontrolled sidings, the best signal indication to enter them typically requires restricted speed since they are other than a main track, though there are a couple of extra rules that apply to sidings (such as not leaving sidings blocked without permission).

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. Timetables showed this as a manual interlocking controlled by an operator at Norden. When Norden was closed, this became CTC under the dispatcher's control. Other than transferring who and where the control operator was, I don't know of any actual changes between Manual Interlocking and CTC. I think the reason the terminology was different had to do with union rules. If it was called CTC, a dispatcher would have to be assigned. By calling it a Manual Interlocking, a local operator could be assigned.

Absolute Permissive Block (APB) is another interesting one. The GCOR 1st edition of 1985 defines it as "A designated section of track or tracks within which the movement of trains will be governed by block signals, whose indications supersede the superiority of trains. The block signals may be controlled automatically or manually." The APB systems I am aware of basically operated as an automatic interlocking. I'm not sure the manual operation refers to a control operator as this would make it a CTC island or manual interlocking. More likely this refers to the train crew being able to push a button to request the route. The controlling signals are not block signals with number plates. There were special instructions on how to flag through APB if a train could not get a proceed signal.

All ABS systems do not have the same signal arrangement, because the intermediate signal can either be staggered or in "flat pairs" (opposing signals across from each other). 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. Assuming both trains passed their signals at almost the same time, they would both meet at the signal between them (which would be red) at full speed with no prior warning. According to schedule, train order, DTC or TWC - one of the opposing trains should not have authority to occupy the main track yet the ABS might not be able to prevent a head on collision. One of the options was to change the intermediate signals between sidings to staggered pairs. In the event opposing trains did unexpectedly meet, each would receive a red signal and the distance between the signals would give them an opportunity to stop short. At sidings, the signal system might incorporate an extra approach circuit (correct term is overlap?) which would change the status of the approaching signal block to occupied before actually entering the block.

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. Any opposing train would receive a red signal requiring them to stop before leaving the siding, making a potential head-on collision less likely. As a train proceeded between sidings, the signals would drop to red behind them preventing a following train from following them except at restricted speed. When all the blocks between sidings were unoccupied, the ABS system would reset and clear the signals in both directions for the next train. Between siding switches, an overlap circuit caused the opposing signals at both ends of the siding to drop to red a the train approached. Because a train shouldn't encounter an unexpected red signal between sidings, an approach warning was not required. Approaching a signal at a siding which marked the end of the permissive block, originally a distant signal was installed to give an approach indication if the signal at the siding was red. This caused confusion so the ICC outlawed having a signal within a block that did not indicate the condition of the approaching block. This is why the signals approaching a siding switch were modified to have two arms to give a clear, approach or stop indication.

The Santa Fe's Second District to San Diego had CTC installed during World War II. But - Santa Fe called it TCS (Traffic Control System). The only apparent difference was that a different company supplied the signal equipment. Was CTC trademarked by US&S? There used to be abandoned concrete signal footings that indicated the original signal arrangement had been staggered pairs. Was this originally ABS? By the early 1980s, the CTC/TCS system had a direction of traffic. If the dispatcher cleared an absolute signal, all the opposing signals went to red like a permissive block system. This system had searchlight signals and would allow a following train. If there was a failure resulting in a track light, the dispatcher could not reverse the direction of traffic. A following train only had to flag the block with the track light, but a train in the opposite direction had to flag every block between the absolute signals. All this signal system has since been replaced by San Diego Northern. I'm not sure if all CTC systems used a direction of traffic or permissive block arrangement. Since the dispatcher knows if the track between absolutes is occupied or not, and a train can't pass an absolute red without verbal permission, I'm not sure it serves any real purpose other than as a back up for safety. The change of color light signals to approach lighting has made it somewhat difficult to determine where trains are and what the signal system is doing.

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. Doesn't the commuter train out of Salt Lake use an early version of this?

Is anyone aware of any remaining APB or has it been upgraded, redesignated or abandoned? Likewise is their any of the ABS left with two-indication intermediates and permissive block? I think SP replaced many of the semaphores on the Siskiyou line with a different ABS system that had three aspects, and CORP either continued the upgrade or abandoned some of the remaining system. Are there any lower quadrant semaphores systems left? Line to Phoenix?