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Varieties of ABS
Author: SP5103
Date: 11-10-2013 - 18:04
As can be seen from our discussions here, there isn't just one "ABS" system and they varied widely, and no doubt some did not follow the AAR standard plans.
First - the signals themselves might be lower quadrant semaphore, upper quadrant semaphore, color light, searchlight, position light, etc.
Second - the actual wiring and operating theories can vary.
The simplest ABS system is for one direction signaling on double track. The battery track circuit feeds from behind the signal. If the track is shunted/occupied, the track relay drops out and the signal goes to red. A red signal changes the polarity of the battery feed to the next signal in advance. Assuming that block is not occupied, the (polar?) track relay that is opposite the normal polarity will cause the signal to display a hard yellow approach. For just three signal aspects, this can be done through the rails without line wires.
ABS for single track gets far more complicated, and does require the use of line wires. Here is where things get confusing. A train that passes the head block signal (leaving signal of a siding) will cause all the opposing signals to "tumble down" up to the next siding and give a red signal to any opposing movements. If the circuits do not allow following trains, then two aspect signals can be used except for ones approaching sidings and head blocks, but as the red signals drop behind it they will not clear until the train is at the next siding.
If all the signals are at least three aspect and can display a hard yellow approach, then the circuits will usually allow a following train. Now, if you simply remove the number plate or add an "A" plate to the head block signals, you now have an Absolute Permissive Block system (using the signal definition, not railroad). Now that it is required to have a proceed signal to leave a siding, the rules must allow for some manner to flag the blocks to the next siding in event of a failed track circuit showing a false occupancy.
If there is only one intermediate signal between sidings, then they have to be a staggered pair or have overlaps in the unlikely event two opposing trains enter the block at the same time meeting unexpectedly at a red signal. If the ABS system does not use permissive block, then it should have either overlap circuits or staggered signals capable of displaying a hard yellow approach aspect.
When you think about it, only the head block signal of a (Absolute) Permissive Block need display a stop indication to keep an opposing train from leaving the siding station, the intermediates could show actual occupancy. I'm really not sure exactly how all the various systems, but they were clearly not all the same "ABS".
APB per the SP definition was used in at least three places I know of. 1) Between Beburg and Greton on the Tillamook branch just east of Portland where the Oregon Electric had trackage rights. 2) Between Wendel and Flanigan on the Modoc. 3) Oxnard and Montalvo on the Coast Line (1966 Special Instructions) - I'm sure there were more.
The Modoc book says that when SP abandoned the line geographic south of Flanigan in exchange for WP trackage rights, SP first installed an Interlocking controlled by the Wendel operator because they did not want to assign an operator to issue train orders at Flanigan. In 1977 this was changed to APB "but the system remained basically the same". Probably nothing more than a approach circuit or push button for crews was installed.
Few people probably recall that the first section of the modern San Diego Trolley was built on the old SD&AE as single track with directional passing sidings or short stretches of double track. This was because only state and local money was available, federal grants weren't interested. This line had some kind of ABS with automatic routing at switches. As I remember, they were power switches, not spring switches. A friend that was one of the original planners complained that the estimates for full CTC were about the same. I assume this was some form of APB on the single track stretches that also took care of lining the switches. I know some of the signal masts had push buttons so operators could cancel a route. The original headways were supposed to be 15 minutes at peak, but this only lasted a few days before they went to 20 minutes. After being embarrassed, the feds showed up with the money to double track the line soon after phase 1 was completed. Typical of government projects, many of the concrete catenary poles and even substations had to be moved for the double track, despite that being the next priority. There was/is one section under a freeway bridge that the freights can only use one of the tracks due to clearances. Power crossovers automatically line up. I recall being told a switch on the wire tripped by the pantograph or lack thereof triggers the route. They also had issues when they added an "express" circuit. I don't think the Trolley has run express trains except for special events, but deadhead equipment and the nightly freights use this system approaching grade crossings. The grade crossing signals immediately after a station platform will normally not trip due to an approaching movement, but will either time out or get some kind of signal to drop the gates when the trolley is prepared to depart. I guess they had a couple of good smash ups before they got this right.
The Sprinter on the old Escondido branch uses a similar single track and double track/directional siding system with power switches and ABS. They have block signals to leave a station which are wired into any adjacent grade crossings. They only get a proceed signal after the time out and gates are dropping. Not sure how they handle Pacific Sun running at night which also requires extended platform ramps for the floor entry Sprinter DMUs to raise to clear the freights. While the Sprinter and Sand Diego Trolley are both LRT systems, part of their system is used for freight on a time separation basis and must meet most FRA requirements.
I have absolutely no idea how the modern signal systems work that use an electronic signal through the rails between signals as to if permissive block or a direction of traffic is used. As previously stated, approach lighting makes it difficult to observe the operating theory most of the time.
First - the signals themselves might be lower quadrant semaphore, upper quadrant semaphore, color light, searchlight, position light, etc.
Second - the actual wiring and operating theories can vary.
The simplest ABS system is for one direction signaling on double track. The battery track circuit feeds from behind the signal. If the track is shunted/occupied, the track relay drops out and the signal goes to red. A red signal changes the polarity of the battery feed to the next signal in advance. Assuming that block is not occupied, the (polar?) track relay that is opposite the normal polarity will cause the signal to display a hard yellow approach. For just three signal aspects, this can be done through the rails without line wires.
ABS for single track gets far more complicated, and does require the use of line wires. Here is where things get confusing. A train that passes the head block signal (leaving signal of a siding) will cause all the opposing signals to "tumble down" up to the next siding and give a red signal to any opposing movements. If the circuits do not allow following trains, then two aspect signals can be used except for ones approaching sidings and head blocks, but as the red signals drop behind it they will not clear until the train is at the next siding.
If all the signals are at least three aspect and can display a hard yellow approach, then the circuits will usually allow a following train. Now, if you simply remove the number plate or add an "A" plate to the head block signals, you now have an Absolute Permissive Block system (using the signal definition, not railroad). Now that it is required to have a proceed signal to leave a siding, the rules must allow for some manner to flag the blocks to the next siding in event of a failed track circuit showing a false occupancy.
If there is only one intermediate signal between sidings, then they have to be a staggered pair or have overlaps in the unlikely event two opposing trains enter the block at the same time meeting unexpectedly at a red signal. If the ABS system does not use permissive block, then it should have either overlap circuits or staggered signals capable of displaying a hard yellow approach aspect.
When you think about it, only the head block signal of a (Absolute) Permissive Block need display a stop indication to keep an opposing train from leaving the siding station, the intermediates could show actual occupancy. I'm really not sure exactly how all the various systems, but they were clearly not all the same "ABS".
APB per the SP definition was used in at least three places I know of. 1) Between Beburg and Greton on the Tillamook branch just east of Portland where the Oregon Electric had trackage rights. 2) Between Wendel and Flanigan on the Modoc. 3) Oxnard and Montalvo on the Coast Line (1966 Special Instructions) - I'm sure there were more.
The Modoc book says that when SP abandoned the line geographic south of Flanigan in exchange for WP trackage rights, SP first installed an Interlocking controlled by the Wendel operator because they did not want to assign an operator to issue train orders at Flanigan. In 1977 this was changed to APB "but the system remained basically the same". Probably nothing more than a approach circuit or push button for crews was installed.
Few people probably recall that the first section of the modern San Diego Trolley was built on the old SD&AE as single track with directional passing sidings or short stretches of double track. This was because only state and local money was available, federal grants weren't interested. This line had some kind of ABS with automatic routing at switches. As I remember, they were power switches, not spring switches. A friend that was one of the original planners complained that the estimates for full CTC were about the same. I assume this was some form of APB on the single track stretches that also took care of lining the switches. I know some of the signal masts had push buttons so operators could cancel a route. The original headways were supposed to be 15 minutes at peak, but this only lasted a few days before they went to 20 minutes. After being embarrassed, the feds showed up with the money to double track the line soon after phase 1 was completed. Typical of government projects, many of the concrete catenary poles and even substations had to be moved for the double track, despite that being the next priority. There was/is one section under a freeway bridge that the freights can only use one of the tracks due to clearances. Power crossovers automatically line up. I recall being told a switch on the wire tripped by the pantograph or lack thereof triggers the route. They also had issues when they added an "express" circuit. I don't think the Trolley has run express trains except for special events, but deadhead equipment and the nightly freights use this system approaching grade crossings. The grade crossing signals immediately after a station platform will normally not trip due to an approaching movement, but will either time out or get some kind of signal to drop the gates when the trolley is prepared to depart. I guess they had a couple of good smash ups before they got this right.
The Sprinter on the old Escondido branch uses a similar single track and double track/directional siding system with power switches and ABS. They have block signals to leave a station which are wired into any adjacent grade crossings. They only get a proceed signal after the time out and gates are dropping. Not sure how they handle Pacific Sun running at night which also requires extended platform ramps for the floor entry Sprinter DMUs to raise to clear the freights. While the Sprinter and Sand Diego Trolley are both LRT systems, part of their system is used for freight on a time separation basis and must meet most FRA requirements.
I have absolutely no idea how the modern signal systems work that use an electronic signal through the rails between signals as to if permissive block or a direction of traffic is used. As previously stated, approach lighting makes it difficult to observe the operating theory most of the time.
