Railway Signalling and Operations FAQ: Absolute-Permissive Block

Railway Signalling and Operations

Absolute-Permissive Block


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Acknowledgement

I am indebted to Al Krug for his extensive understanding of APB signalling and his help in allowing me, after several iterations, finally to understand it myself. He "keeps me honest", as the phrase goes. This description shows his influence and may introduce a bit too much of a Burlington Northern Santa Fe bias as a result.

Jon Roma saved Al's post when my fingers were too incoordinated to accomplish the same.

Carsten Lundsten provided many comments on Santa Fe practice.


Notes

APB stands for Absolute-Permissive Block. The term APB must not be confused with the general terms "absolute block" and "permissive block", which do not describe a method of operation. Note further that some railroads (including Al's BNSF) call this operational scheme "ABS". I find this term ambiguous, as it is inconsistently applied by N.Am. railroads. I will use APB in this document.

I will use the term "head-block signal". This term will designate a specific signal type (operationally) that appears only in APB. BNSF, at least, calls these signals "absolute signals", which they certainly are. But "absolute signal" is also a general term - other signal types fit the definition of "absolute". To be clear, I'll use "head-block signal" here.

This description is heavily influenced by Al Krug and BNSF practice. If you know of other applications of APB, please email me! Anyone with firsthand experience with Conrail's installation of APB on the former PRR main line across Indiana and western Ohio, please email me!

Finally, I intend "soon" to replace the ASCII drawings with gifs. Please be patient. Bug me if I forget.


Absolute-Permissive Block

Absolute-Permissive Block is best described as a "poor-man's CTC". It is approximately standard automatic block signalling, except that certain signals are defined as being absolute, and that switches are thrown manually. There are no "interlockings". I'll start with a track diagram with 2 trains, AAAAA and BBBBB. The signals with "A" plates are absolute signals, where a red light means stop-and-stay. (They may or may not have number plates also, depending on the railroad.) All other signals have a number plate; thus, they are permissive signals. Invented milepost numbers are provided for permissive signals. Just for fun, the track at XXXXX is temporarily out of service, by Bulletin Order.


    WEST ABEL    EAST ABEL                             WEST BAKER     EAST BAKER
      HB  ---------      181        193       205       HB    -<BBBBB---       229
  G--A|  /         \ R--#|      Y--#|     G--#|      G--A|   /          \  G--#|
-------------AAAAA>-------------------------------------------XXXXXXXXXX---------------
      |#--R              |A--G      |#--G     |#--G      |#--G                 |A--G
    170                 HB        192       204        216                    HB
On BNSF (? and other railroads), the automatic signals respond only to block occupancy of the main track block immediately in advance of that signal (plus the aspect of the signal in advance). I.e., signal 181 is red because of train AAAAA. If the switch at EAST ABEL is thrown reverse, 181 will not change its aspect because it continues to look only at the main track. [Does anyone know of other arrangements??] Further, if AAAAA were not there, reversing that switch would cause 181 to drop to red.

Note that the head-block signals are beyond the trailing-point switches. The engineers must stop their trains not at that signal, as with "plain vanilla" ABS, but prior to the clearance point, so that the main or siding track is not fouled.

Finally, both trains are stopped, and the engineers of both trains are staring at green signals. This may appear to be strange at first, until the reader realizes that they are staring at head-block signals. The engineers must have permission to pass these signals (making this like manual block superimposed on automatic block).

Let's say the dispatcher desides to move train BBBBB first. Train BBBBB will be called (e.g., on the radio) and a train order (or modern equivalent) will be transmitted. The train order allows BBBBB to pass the (absolute) HB signal at WEST BAKER, proceed into ABEL siding and around train AAAAA. BBBBB lines the WEST BAKER switch:


    WEST ABEL    EAST ABEL                             WEST BAKER     EAST BAKER
      HB  ---------      181        193       205       HB    -<BBBBB---       229
  G--A|  /         \ R--#|      Y--#|     G--#|      G--A|   /          \  G--#|
-------------AAAAA>-----------------------------------------/-XXXXXXXXXX---------------
      |#--R              |A--G      |#--G     |#--Y      |#--R                 |A--G
    170                 HB        192       204        216                    HB

The view of what action the train order should allow varied among railroads, and the wording of the train order no doubt varied accordingly. Some railroads' train orders gave permission for the train to pass the fouling point of the switch at the end of the siding. In such cases, one's permission to pass the HB signal is only implicit. Other railroads did not mount the absolute and permissive signal together (e.g., the 181 and the nearby HB signal), but rather mounted the head-block signal in the rear of the switch, and their orders gave permission to pass the signal. Mounting the signals separately was needed on all railroads at locations where the HB signals protected a crossing instead of the end of a siding.

Train BBBBB will start pulling out of the siding. Soon, the train will pass the HB signal at WEST BAKER, causing it to drop to red:


    WEST ABEL    EAST ABEL                             WEST BAKER     EAST BAKER
      HB  ---------      181        193       205       HB    BBB-------       229
  G--A|  /         \ R--#|      Y--#|     G--#|      R--A|   B          \  G--#|
-------------AAAAA>------------------------------------<BBBBB-XXXXXXXXXX---------------
      |#--R              |A--G      |#--G     |#--Y      |#--R                 |A--G
    170                 HB        192       204        216                    HB
The fact that the train has passed the absolute (head-block) signal establishes an absolute-permissive block between the head-block signals. This means that there is an absolute block for opposing movements, and a permissive block for following movements. The opposing head-block signal (EAST ABEL) drops to Stop.

The train passing the westward head-block signal causes a tumble-down of the opposing ABS signals in most (?all) APB installations. The (electrical) signal to drop the opposing HB signal to red must be passed from one HB to the other anyway; one may as well take advantage of its availability to drop all intermediate signals as well. Theoretically it is not needed and is only an "added safety feature", since theoretically trains should be adequately controlled at the HB signals. However, the "added safety feature" comes "free", so to speak. At any rate, 204, then 192, then the HB at EAST ABEL tumble down to red:


    WEST ABEL    EAST ABEL                             WEST BAKER     EAST BAKER
      HB  ---------      181        193       205       HB    BBB-------       229
  G--A|  /         \ R--#|      Y--#|     G--#|      R--A|   B          \  G--#|
-------------AAAAA>------------------------------------<BBBBB-XXXXXXXXXX---------------
      |#--R              |A--R      |#--R     |#--R      |#--R                 |A--G
    170                 HB        192       204        216                    HB

Train BBBBB proceeds along the single-track line, approaching signal 181. I assume that, prior to leaving WEST BAKER, train BBBBB will stop and reset the switch to normal; but the rules may vary from railroad to railroad. I also assume that, as the train leaves blocks, signals will reset to Y, then G.


    WEST ABEL    EAST ABEL                             WEST BAKER     EAST BAKER
      HB  ---------      181        193       205       HB    ----------       229
  G--A|  /         \ R--#|      R--#|     Y--#|      G--A|   /          \  G--#|
-------------AAAAA>----------<BBBBB---------------------------XXXXXXXXXX---------------
      |#--R              |A--R      |#--G     |#--G      |#--G                 |A--G
    170                 HB        192       204        216                    HB

Train BBBBB sees a red signal at 181. However, this is an automatic, not a controlled signal. The aspect on 181 is either Stop-and-Proceed, or Restricted Proceed, depending on the railroad. The train will pass the 181 signal, pulls up to the EAST ABEL switch, one of the crew members throws the switch, and the train proceeds into the siding:


    WEST ABEL    EAST ABEL                             WEST BAKER     EAST BAKER
      HB  ------<BB      181        193       205       HB    ----------       229
  G--A|  /         B R--#|      Y--#|     G--#|      G--A|   /          \  G--#|
-------------AAAAA>-BBBB--------------------------------------XXXXXXXXXX---------------
      |#--R              |A--G      |#--G     |#--G      |#--G                 |A--G
    170                 HB        192       204        216                    HB

In the meantime, the dispatcher may have given AAAAA permission to proceed east once BBBBB has cleared the EAST ABEL switch. Rules may vary, and on any given railroad, the dispatcher may have to wait until AAAAA reports clear (calls the dispatcher and tells the Dspr that it, AAAAA, is in the siding and the EAST ABEL switch is again normal) before giving AAAAA permission to proceed. Basically, it depends on whether the railroad wants to be "lax", and allow the engineers to determine when they may proceed, or "strict", and leave that decision in the hands of the dispatcher. At the time AAAAA is ready to proceed, we stand as follows:


    WEST ABEL    EAST ABEL                             WEST BAKER     EAST BAKER
      HB  -<BBBBB--      181        193       205       HB    ----------       229
  G--A|  /         \ R--#|      Y--#|     G--#|      G--A|   /          \  G--#|
-------------AAAAA>-------------------------------------------XXXXXXXXXX---------------
      |#--R              |A--G      |#--G     |#--G      |#--G                 |A--G
    170                 HB        192       204        216                    HB
AAAAA will make sure the EAST ABEL switch is normal, then it will pass the EAST ABEL HB signal, causing a tumble-down of the westward signals:

    WEST ABEL    EAST ABEL                             WEST BAKER     EAST BAKER
      HB  BBBBB----      181        193       205       HB    ----------       229
  G--A|  B         \ R--#|      R--#|     R--#|      R--A|   /          \  G--#|
-------<B-------------AAAAA>----------------------------------XXXXXXXXXX---------------
      |#--R              |A--G      |#--G     |#--G      |#--G                 |A--G
    170                 HB        192       204        216                    HB


Details

Head-Block Signal Placement

[Jon Roma] The alternate head-block signal placement in fact exists on the IC: the HB signals are placed on the frog, instead of point, side of the switch. In this case, the purpose is to give spring switch indication for departing trains.

This type of installation is used where the spring switch has a point lock. To prevent a sharp flange or other abnormality from opening a spring switch during a main line move and causing a derailment some spring switches have an electro-mechanical point locking mechanism. To trail through the spring switch and force the points over, this lock must be de-activated. Thus one needs signals protecting the switch to tell you the lock is off and it is OK to trail through.

[Mark Bej] In particular, I've heard of head-block signals before the clearing point when the control location was a crossing of a foreign line at grade, instead of a switch. E.g. (and simplifying intermediate signals):


                                        ---|
---------------                          | |   HB                  HB  -------
               \           O--|          O | O--|    O--|       O--A| /
-------------------------------------------+----------------------------------
                 |A--O        |--O    |--O | O          |--O
                 HB                   HB   | |
                                           |---


Example Circuit

An example circuit may be found here on Jon Roma's site.


Variants

Illinois Central

[Jon Roma] Spring switch signaling: there are several examples on the Illinois Central. Signal 181 would have a lunar white marker under the main head. The marker only lights when signal 181 shows red and when the spring points are in proper normal or reverse position. This indicates that the train doesn't have to stop to check the spring points.

Burlington Northern

[Al Krug] [BNSF] has a CTC line where only one end of each siding is a control point and only that end had a power switch. The other end has a spring switch. The ABS signal governing movement over the spring switch on our old CTC installation had a lunar light under it that the dispatcher could light to tell a train to hand-operate the spring switch and take the siding.

[BNSF] now has "block overlap" as indicated by signs a couple of thousand feet up the main along the sidings. If we are to meet a train at this station by track warrant then we must stop before the overlap sign to prevent early tumble down of the opposing train's signals. Before this overlap was installed I do not know if the normal tumble down of the original system occurred when one passed the HB or the switch fouling point.

Santa Fe

Someone mentioned exit dwarf signals that show red if at least 3 mainline blocks are not clear. Please tell me who you are.

[Carsten S. Lundsten] reports that ATSF places the exit signal on the frog side of the switch, controlling the main track. There are separate signals governing the siding. The main track signals are permissive, the siding signals absolute. The arrangement would be as follows:


    WEST ABEL    EAST ABEL                             WEST BAKER     EAST BAKER
          o-|                                                   o-|
        -----------      181        193       205             --------------       229
       /G--#|   |-o\ G--#|      G--#|     G--#|              /G--#|      |-o\  G--#|
---------------------------------------------------------------------------------------
   |#--G        |#--G               |#--G     |#--G      |#--G           |#--G
 170           HB                 192       204        216              HB
Siding switches are spring switches, and the signal governing the facing-point direction (e.g., signals 170, 181, 216, 229 on the diagram above) include a white (?illuminated) "S" for "take siding". When the switch is thrown, the signal changes to flashing red.

The dwarf signals governing siding exit are not approach-lit, in contrast to most of the other signals, and show green by default. If any of the 3 main track blocks in the rear of the dwarf are occupied, the dwarf will show red, thus indicating to the train on the siding that it is unsafe to pull out. E.g., In the drawing above, if the 192-204 block, 204-216 block, or 216-HB block is occupied, the dwarf at EAST BAKER will show red.


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Absolute-Permissive Block


Mark D. Bej, M.D.
bejm@ccfadm.eeg.ccf.org
+216-444-0119
1997.04.08