BlockOccupancy
BK Block Occupancy
Introduction
They appear to have been installed by a variety of railroads from about 1940 to the early 1960's. It appears that both GRS and US&S used them.
Overview of "BK" indications as observed on certain ex-Southern Railway territories of Norfolk Southern. (Originally by edwells4915)
Since I think I'm the one who opened this can of worms, I'll have a shot at describing what I know as BK-type indications. Bear in mind that my experience is not as a signalman, but as a programmer working with a stream of indication bits, relying on logic described by the signal department. Also, keep in mind that what I'm familiar with relates specifically to GRS and Harmon field and office equipment on former Southern Railway territories - the current Atlanta North district, Atlanta South district, the North Alabama territory of the NS Alabama Division (Birmingham-Sheffield), and the East End (Birmingham-Atlanta) of the Alabama Division. I don't have any direct knowledge of CSX practice. That being said, let me see if I can describe how these territories worked.
Examples
First, BK-type indications were used between siding switches as well as between the stations. The only place (in general) that we had AK-type approach occupancy indications was on approach tracks entering CTC territory from "dark" territory. Below is a generalized drawing of a siding where BK-type indications are present.
A B C
--+----------------+-- -+-----
/ A:2RBK B:2LBK \ / C:2RBK
----+-----+----------------+-----+---------------+----+-----
A:1LBK 1TK A:1RBK B:1LBK 1TK B:1RBK C:1LBK 1TK C:1RBK
In the diagrams that follow, I'm not going to try to draw switches, just to keep things simple. Also, I'm using "." as place holders, to keep text from arbitrarily getting left justified...
(1) Location:...........A......................B.................C....... +-----------------+-----+----------------+----+------------+----+----- ..0.............1....0......0...........1...0....0........1...0...0... 1RBK..........1LBK. 1TK...1RBK.......1LBK..1TK..1RBK....1LBK.1TK.1RBK ................0...........0..........0..........0.......0........0 ..............1RGK........1LGK.......1RGK.......1LGK....1RGK.....1LGK
OK, above is basic diagram. If you want to think of switches at A,B,C, fine - doesn't really change anything. 1s and 0s indicate the value of the BK indication directly below. Assume all signals at stop, all tracks unoccupied, and that the last cleared route was toward the right.
(2) Location:...........A......................B.................C....... +-----------------+-----+----------------+----+------------+----+----- ..0.............1....0......0...........1...0....0........1...0...0... 1RBK..........1LBK. 1TK...1RBK.......1LBK..1TK..1RBK....1LBK.1TK.1RBK ................1...........0..........1.........0........1.......0... ..............1RGK.>......1LGK.......1RGK>......1LGK....1RGK >...1LGK
In (2), the DS has cleared a route for a "right-bound" train from A through C. At this point, the only change is that the 1RGK bits go from 0 to 1 at A, B, and C.
(3) Location:...........A......................B.................C....... =======T1=========> +-----------------+-----+----------------+----+------------+----+----- ..1.............1....0......0...........1...0....0........1...0...0... 1RBK..........1LBK. 1TK...1RBK.......1LBK..1TK..1RBK....1LBK.1TK.1RBK ................1...........0..........1.........0........1.......0... ..............1RGK>.......1LGK.......1RGK>......1LGK....1RGK>....1LGK
In (3), a train, T1, occupies the block on the left side of A from "off the page" on the left. Note that the 1RBK on the very left side of the page is now 1. This bit, logically ANDed with A:1LBK, indicates occupancy of the block.
(4) Location:...........A......................B.................C....... ====================T1===================> +-----------------+-----+----------------+----+------------+----+----- ..1.............1....1......1..........1....0....0........1...0...0... 1RBK..........1LBK. 1TK...1RBK.......1LBK..1TK..1RBK....1LBK.1TK.1RBK ................0...........0..........1..........0.......1........0... ..............1RGK........1LGK.......1RGK>......1LGK....1RGK>....1LGK
In (4), T1 has OS'd at A, and has entered the block between A and B. It now occupies the OS at A (A:1TK) and the blocks on either side. Note that A:1TK and A:1RBK are now 1
(5) Location:...........A......................B.................C....... ...................===============T1=====> +-----------------+-----+----------------+----+------------+----+----- ..0.............1....1......1...........1...0....0........1...0...0... 1RBK..........1LBK. 1TK...1RBK.......1LBK..1TK..1RBK....1LBK.1TK.1RBK ................0...........0.........1...........0......1........0... ..............1RGK........1LGK.......1RGK>......1LGK....1RGK>....1LGK
In (5), the rear of T1 has cleared the block to the left of A. The 1RBK on the left has "dropped", i.e. is 0. The block now indicates "unoccupied".
(6) Location:...........A......................B.................C....... .........................===============T1=================> +-----------------+-----+----------------+----+------------+----+----- ..0.............1....0......1...........1...1....1........1...0...0... 1RBK..........1LBK. 1TK...1RBK.......1LBK..1TK..1RBK....1LBK.1TK.1RBK ...............0...........0..........0..........0.......1........0. ..............1RGK........1LGK.......1RGK.......1LGK....1RGK>....1LGK
In (6),the train has passed through interlocking at B and occupies the block between B and C. Rear end still occupies the block between A and B. Note A:1RBK still 1, B:1TK and B:1RBK are now 1.
(7) Location:...........A......................B.................C....... ...............................................==============T1======> +-----------------+-----+----------------+----+------------+----+----- ..0.............1....0......0...........1...0....1........1...1...1... 1RBK..........1LBK. 1TK...1RBK.......1LBK..1TK..1RBK....1LBK.1TK.1RBK ...............0...........0..........0..........0.......0........0... ..............1RGK........1LGK.......1RGK.......1LGK....1RGK.....1LGK
In (7), the rear of the train has cleared OS circuit at B, and has occupied OS at C and the block on the right of C.
(8) Location:...........A......................B.................C....... .................................................................=T1=> +-----------------+-----+----------------+----+------------+----+----- ..0.............1....0......0...........1...0....0........1...0...1... 1RBK..........1LBK. 1TK...1RBK.......1LBK..1TK..1RBK....1LBK.1TK.1RBK ...............0...........0..........0..........0.......0........0... ..............1RGK........1LGK.......1RGK.......1LGK....1RGK.....1LGK
In (8), the train has cleared the block on the left of C, and the OS at C. B:1RBK and C:1TK return to 0.
(9) Location:...........A......................B.................C....... +-----------------+-----+----------------+----+------------+----+----- ..0.............1....0......0...........1...0....0........1...0...0... 1RBK..........1LBK. 1TK...1RBK.......1LBK..1TK..1RBK....1LBK.1TK.1RBK ...............0...........0..........0..........0.......0........0... ..............1RGK........1LGK.......1RGK.......1LGK....1RGK.....1LGK
In (9),the train has cleared the block to the right of C. We're now back at our original state: all signals at stop, all blocks unoccupied.
Had enough? thought not... hang on... Now the dispatcher needs to clear a route for a train entering on the right. First, the BKs have to go through some transition...
(10) Location:...........A......................B.................C....... +-----------------+-----+----------------+----+------------+----+----- ..0.............0....0......0..........0....0.....0.......0...0....0... 1RBK..........1LBK. 1TK...1RBK.......1LBK..1TK..1RBK....1LBK.1TK.1RBK ...............0...........0..........0..........0.......0........0... ..............1RGK........1LGK.......1RGK.......1LGK....1RGK.....1LGK
In 10, the plant is in a transitional state ... the 1LBK bits at A,B, & C go to 0. 1RBKs remain at 0. In the real world, this doesn't necessarily happen all at once. From what I've seen, the BK bits change over (traffic direction reverses) on a station by station basis as the dispatcher clears the route one CP at a time.
(11) Location:...........A......................B.................C....... +-----------------+-----+----------------+----+------------+----+----- ..1.............0....0......1..........0....0.....1.......0...0....1... 1RBK..........1LBK. 1TK...1RBK.......1LBK..1TK..1RBK....1LBK.1TK.1RBK ...............0...........0..........0..........0.......0........0... ..............1RGK........1LGK.......1RGK.......1LGK....1RGK.....1LGK
In (11), we're ready to clear "left-bound" signals...
(12) Location:...........A......................B.................C....... +-----------------+-----+----------------+----+------------+----+----- ..1.............0....0......1..........0....0.....1.......0...0....1.. 1RBK..........1LBK. 1TK...1RBK.......1LBK..1TK..1RBK....1LBK.1TK.1RBK ...............0...........1..........0..........1.......0........1.. ..............1RGK.......<1LGK.......1RGK......<1LGK....1RGK....<1LGK
In (12), left-bound signals cleared, route cleared from C through A...
(13) Location:...........A......................B.................C....... ...........................................................<===T2===== +-----------------+-----+----------------+----+------------+----+----- ..1.............0....0......1..........0....0.....1.......0...1....1.. 1RBK..........1LBK. 1TK...1RBK.......1LBK..1TK..1RBK....1LBK.1TK.1RBK ...............0...........1..........0..........1.......0........0.. ..............1RGK.......<1LGK.......1RGK......<1LGK....1RGK.....1LGK
In (13), train T2 has entered the interlocking heading left...
(14) Location:...........A......................B.................C....... ...............................................<===T2============ +-----------------+-----+----------------+----+------------+----+----- ..1.............0....0......1..........0....0.....1.......1...1....1.. 1RBK..........1LBK. 1TK...1RBK.......1LBK..1TK..1RBK....1LBK.1TK.1RBK ...............0...........1..........0..........1.......0........0.. ..............1RGK.......<1LGK.......1RGK......<1LGK....1RGK.....1LGK
In (14), T2 has entered the block between B & C. C:1LBK goes to 1. The 1LBK that would be at D (if I'd included D) has gone to 0. I'll show a couple more states, without comment... you should have the idea by now.
(15) Location:...........A......................B.................C....... .........................<===T2============================ +-----------------+-----+----------------+----+------------+----+----- ..1.............0....0......1..........1....1...1.........1...0....1. 1RBK..........1LBK. 1TK...1RBK.......1LBK..1TK.1RBK.....1LBK.1TK.1RBK ...............0...........1..........0..........0.......0........0.. ..............1RGK.......<1LGK.......1RGK......1LGK.....1RGK.....1LGK (16) Location:...........A......................B.................C....... .<===T2=================================== +-----------------+-----+----------------+----+------------+----+----- ..1.............1....1......1..........1....0...1.........0...0....1.. 1RBK..........1LBK. 1TK...1RBK.......1LBK..1TK.1RBK.....1LBK.1TK.1RBK ...............0...........0..........0..........0.......0........0.. ..............1RGK........1LGK.......1RGK......1LGK.....1RGK.....1LGK (17) Location:...........A......................B.................C....... .<===T2============ +-----------------+-----+----------------+----+------------+----+----- ..1.............1....0......1..........0....0...1.........0...0....1. 1RBK..........1LBK. 1TK...1RBK.......1LBK..1TK.1RBK.....1LBK.1TK.1RBK ...............0...........0..........0..........0.......0........0.. ..............1RGK........1LGK.......1RGK......1LGK.....1RGK.....1LGK (18) Location:...........A......................B.................C....... +-----------------+-----+----------------+----+------------+----+----- ..1.............0....0......1..........0....0...1.........0...0....1. 1RBK..........1LBK. 1TK...1RBK.......1LBK..1TK.1RBK.....1LBK.1TK.1RBK ...............0...........0..........0..........0.......0........0.. ..............1RGK........1LGK.......1RGK......1LGK.....1RGK.....1LGK
... and back to the original state...
Notes and Caveats
One thing to note is that in the above state, signals can only be cleared for movement to the left. If I want to clear the leftward signal at B (B:1LGK), the 1RBK at A has to be 1. If there are intermediate signals between A & B, then it's also possible for the dispatcher to see a leftward "clear" at B with the B:1LBK at 1, although the aspect in the field will be less permissive than "clear". With regard to the transitional state, when the dispatcher is "turning the traffic", it's not always as stable as all the BKs going to 0. For a given block, I've seen both go briefly to 1, then to 0, and then settle out with the appropriate combination for the traffic direction. There may be other variations on this theme, as well. I think it's related to the timing of various relays, and sometimes varies from control point to control point. It's not a problem if you're turning lights on and off on a model board, or coloring track lines on a CRT display, but if you're trying to track a train ID across a territory by associating it with occupancies, it requires some specialized logic to deal with all the flip-flopping of indication bits during these transitions. Also, I can't remember if the dispatcher in this type of system has to "turn the traffic" direction separately from clearing the route, or if requesting the opposing route (assuming occupancies permit) also reverses the traffic direction. I'd like to know, if anyone's familiar with this.
Identifying BK's
I've been thinking about what sort of MCP Plot one might expect if BK-type indications were actually in use. Admittedly, I haven't run across any of these yet - the CSX Manchester Sub and Atlanta Terminal are fairly well-behaved :) However, it seems to me that you should see a plot something like the following:
Given this layout - again, no switches, just to keep the drawing simple. Pretend it's a holdout signal...
+--------------------+---+---------------------------+
................1LBK..1TK..1RBK.......................
Then I'd expect MCP plot patterns to look something like:
Traffic direction >>
1LBK ..................................... ..... ....
1RBK .... ... ...... ..........................
<< Traffic direction
I think the key to recognizing the pattern is that you'll have a pair of bits, one of which will stay on for a long (relatively) period of time, during which the other turns on and off for one or more shorter intervals. When the traffic direction is reversed, the bit that was on for short intervals will now remain on, while the one that was on continuously will begin turning on and off. The above plot indicates three occupants moving from left to right, and then after a transition period, two occupants moving from right to left. In the case of a siding, you should also expect to see a 2RBK or 2LBK, depending on which end of the siding you're looking at. See the diagram at the beginning of this document for details.
Usage
BK approach blocks are a pair of directional logical blocks between two MCP's. While at first they look like normal EAK and WAK blocks, you will start to notice that the approach block in front of the green signal is always red until the train gets past the TK, at which point the arriving approach block will go back to white. The ATCSmon solution to this is to create a single block between the MCP's and in the block properties name it :B and then identify the stations to it's left and right. In MCP Maintenance, identify the proper bit as having a BK indication in both the left and right MCP's.
south north
|----------|--\ Actual Field Conditions /--|----------|
\ /
|----------|----\---|-----NBK-----|-----SBK-----|---/----|----------|
|----------|--\ Track Builder Setup /--|----------|
\ /
|----------|----\---|------------BK-------------|---/----|----------|
X Y
Xenia Yankeetown
Station to Left = X Station to Right = Y Name = :B
Inverted BKs
Background
On the CSX through Pittsburgh, the interlockings that are giving us trouble, are all older interlockings, on the ex. P&LE main. Besides the BK's they also give a zillion route and switch locking indications, but that's another story.
The interlockings are closely spaced, so the ones I've studied so far all have only one block in between. Each interlocking indicates the block by one bit, but contrary to the BK's already described, they go active to indicate a free block. Reversal of direction of traffic is selected by commanding the current "departure" interlocking to flip the direction. So commands for a route through one interlocking is likely to also include a command for the next.
Examples
If we use Ed's diagrams, it looks like this in inactive state, given that last train went left to right: (1)
Location:...........A......................B.................C....... +-----------------+-----+----------------+----+------------+----+----- ..0.............1....0......0..........1....0....0........1...0...0... 1RBK..........1LBK. 1TK...1RBK.......1LBK..1TK..1RBK....1LBK.1TK.1RBK ................0...........0..........0.........0........0.......0... ..............1RGK........1LGK.......1RGK.......1LGK....1RGK.....1LGK
The LBK comes up when a train clears the block, and stays up, provided that another route has not already been cleared to the block. I guess this could be interpreted as "not traffic locked". It doesn't come up after the direction is flipped, only after the block has been occupied and again free. But maybe this is because a route is already "waiting" when the direction flips.
Anyways, when a route is cleared left to right, the BK's flip, so that the RBK's come on, the LBK's disappear, and the signals come up: (2)
Location:...........A......................B.................C....... +-----------------+-----+----------------+----+------------+----+----- ..1.............0....0......1..........0....0....1........0...0...1... 1RBK..........1LBK. 1TK...1RBK.......1LBK..1TK..1RBK....1LBK.1TK.1RBK ................1...........0..........1.........0........1.......0... ..............1RGK>.......1LGK.......1RGK>......1LGK....1RGK>....1LGK
When the train comes on the approach to "A", both BK's are inactive: (3)
Location:...........A......................B.................C....... ==T1=====> +-----------------+-----+----------------+----+------------+----+----- ..0.............0....0......1..........0....0....1........0...0...1... 1RBK..........1LBK. 1TK...1RBK.......1LBK..1TK..1RBK....1LBK.1TK.1RBK ................1...........0..........1.........0........1.......0... ..............1RGK>.......1LGK.......1RGK>......1LGK....1RGK>....1LGK
And the train progresses into interlocking "A": (4)
Location:...........A......................B.................C....... ==========T1===========> +-----------------+-----+----------------+----+------------+----+----- ..0.............0....1......1..........0....0....1........0...0...1... 1RBK..........1LBK. 1TK...1RBK.......1LBK..1TK..1RBK....1LBK.1TK.1RBK ................0...........0..........1.........0........1.......0... ..............1RGK........1LGK.......1RGK>......1LGK....1RGK>....1LGK
And through "A": (5)
Location:...........A......................B.................C.......
==========T1===========>
+-----------------+-----+----------------+----+------------+----+-----
..0.............0....1......0..........0....0....1........0...0...1...
1RBK..........1LBK. 1TK...1RBK.......1LBK..1TK..1RBK....1LBK.1TK.1RBK
................0...........0..........1.........0........1.......0...
..............1RGK........1LGK.......1RGK>......1LGK....1RGK>....1LGK
After the train's tail enters interlocking "A", "A"'s LBK comes on (assuming the previous interlocking has not already set another route towards "A"): (6)
Location:...........A......................B.................C.......
==========T1===========>
+-----------------+-----+----------------+----+------------+----+-----
..0.............1....1......0..........0....1....1........0...0...1...
1RBK..........1LBK. 1TK...1RBK.......1LBK..1TK..1RBK....1LBK.1TK.1RBK
................0...........0..........0.........0........1.......0...
..............1RGK........1LGK.......1RGK.......1LGK....1RGK>....1LGK
Actually the RBK of the block will come up for a second or two when the train clears, then disappear.
After clearing interlocking "A", the dispatcher requests another route. The interlocking locks switches, but the signal indication does not come up as the block towards B is occupied: (7)
Location:...........A......................B.................C.......
==========T1===========>
+-----------------+-----+----------------+----+------------+----+-----
..0.............1....0......0..........0....1....0........0...0...1...
1RBK..........1LBK. 1TK...1RBK.......1LBK..1TK..1RBK....1LBK.1TK.1RBK
................0...........0..........0.........0........1.......0...
..............1RGK>.......1LGK.......1RGK.......1LGK....1RGK>....1LGK
The train clears the block towards "B", and "A"'s BK comes up and the signal clears: (8)
Location:...........A......................B.................C.......
=========T1==========>
+-----------------+-----+----------------+----+------------+----+-----
..0.............1....0......1..........0....1....0........0...1...1...
1RBK..........1LBK. 1TK...1RBK.......1LBK..1TK..1RBK....1LBK.1TK.1RBK
................1...........0..........0.........0........0.......0...
..............1RGK>.......1LGK.......1RGK.......1LGK....1RGK.....1LGK
Usage
-- Main.BrianSwan - 24 Oct 2006