The early adoption of automatic block signals (ABS) underscored the progressive attitude of Milwaukee Road management. However, that adoption was never fully complete. The section of the mainline unused by passenger trains, where freighters bypassed Spokane, was never equipped with ABS (the 1977 Pandora head on collision occurred in this "dark" section; two crew members were killed). And when the Milwaukee gained access to Portland, as a condition of the Burlington Northern merger in 1970, ABS was not installed on the South Line from Tacoma to Chehalis. That subdivision had always been considered branch line, but the introduction of the four Portland trains effectively converted it to dark mainline.
The Joint Line was equipped with ABS, and it is difficult to imagine how such a heavily used section of single track, shared by two first class railroads, could have been operated without it. Indeed, the time and labor saved by the use of ABS reduced Rule 99 to an afterthought: "When the rear of the train is protected by a continuous ABS System, protection against following trains on the same track is not required" (footnote to Rule 99, p. 46 C.C.O.R. 1967).
Automatic Block Signal System Rules
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The Consolidated Code of Operating Rules, Edition of 1967
ABS Operation
In ABS territory, the presence of a train caused the signal governing that block of track to turn red. A train following that train would encounter a yellow "approach" signal to the block preceding the block the first train occupied. This approach indication was the advance warning that the next signal would be a red "stop" indication. Most likely that stop indication was at an "intermediate" signal, for which the ABS rules required that the following train stop, "whistle off," and then proceed at restricted speed ("Proceed prepared to stop short of train, engine, obstruction, or switch not properly lined, looking out for broken rail or anything that may require the speed of a train or engine to be reduced, but not exceeding 20 MPH" Definitions, p. 11, C.C.O.R. 1967). Thus the following train was able to creep up on the caboose of the proceeding train, and sometimes give that train a boost (i.e. shove the proceeding train up the grade - a situation that could cause an anxious conductor of the shoving train to radio his "green" head brakeman to inquire as to whether a "joint" was made with the caboose, "if those knuckles are closed, and pass, and shear off that caboose's angle cock, you'll have it for lunch"). An intermediate, or "stop and go" ABS signal was identified by the presence of a number plate. The other kind of ABS signal, known as an "absolute" (or "head") signal, had no such number plate and if a stop indication was displayed, it could not be passed without permission from the dispatcher (rarely used exceptions not withstanding). These signals were found at the leaving end of a station (aka "siding"). Once a train left a station, and passed this signal, all the opposing intermediate signals, and opposing head signal at the next station, changed to red. This is the feature of the ABS system which prevented trains from meeting head on; it was simply impossible for opposing trains to be between the same two stations.
Rule 513
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The Consolidated Code of Operating Rules, Edition of 1967
Rule 513
Rule 513, known as the "five minute" rule, was key to the operation of ABS. This rule was designed to prevent the situation where a signal "dropped in your face" (either the preceding signal was green, or a signal actually changed from green to red as you approached it). The five minute rule required that a train or engine intending to enter ABS territory wait for five minutes after the lining the switch for that movement. Five minutes was the minimum amount of time necessary to "establish block signal protection" (i.e. to clear two blocks to prevent red blocks "dropping"). And woe to the head brakeman who, on account of rain or snow, did not attend that switch for the full five minutes, for what good would the rule have been if a train, with no warning, suddenly was routed into a collision with his train waiting down its time?
Block and Interlocking Signals
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The Consolidated Code of Operating Rules, Edition of 1967
The Dispatcher as Flagman
As we saw from train orders and the superiority of trains, no inferior movement was permitted to occupy the main track on the time of a superior train. Further, Rule 81 ("Movement of Trains" p. 38 C.C.O.R. 1967) provided: "A main track must not be fouled or occupied without authority, unless protected as prescribed by Rule 99." And as we saw above, the flagging rule, Rule 99, did not apply in ABS territory. These rules underpinned the murky situation where a job like the Valley Owl could get the dispatcher to "flag" their movement.
Block and Interlocking Signals
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The Consolidated Code of Operating Rules, Edition of 1967
Here was the situation: at West Siding yard, its usual haunt, the Owl frequently needed the mainline in order to complete a switching move; for example shuffle a pickup up to the pass or shuffle a setout down from the pass. Yet with six daily schedules on the Joint Line, and everyone running around with green signals, there was rarely a time when some scheduled train was not due. This would normally mean that a local, running extra, or doing yard work, could not legally occupy the mainline. The solution was to call the dispatcher on the phone (a dispatcher's phone, secured by switch lock, was available at both ends of active sidings) and ask for "permission" to open up and come out on the main. While the dispatcher was not authorized to waive Rule 513, he could informally "flag" the movement by letting the crew involved know that nothing was coming. Thus they asked for the dispatcher's permission but received something less, as would have been revealed by any incident investigation. This is how the railroad operated: one can view it as either "safety occasionally first" or that any a priori rule could be trumped by a footnote to Rule 99.
ABS Signal 10 Over 4 at Black River Yard
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View South (Time Table East), Milwaukee Road maintained block signals on B.N. 11th Sub Eastward Main
ABS Signal with G Marker
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View East, Block Signal on approach to Snoqualmie Tunnel