The "Signal Basics" document describes the basic functionality of signaling, advance signaling and some solutions where irregular signaling is required. This document will use this to describe how to design signals for the model railroad, and will use this technique to "reverse engineer" prototype interlocking diagrams.
The signal aspects and indications used in this document are the PRR 1944 signal aspects and indications, simplified for model railroad use. The "reverse engineering" chapter uses interlocking diagrams for PRR's PG interlocking (1947) and OB Interlocking (1946).
The physical look of a signal reflects the signal aspects that it can show. A given signal is usually only capable of displaying a subset of the total signal aspects, and the signal is only equipped with the lamps and control logic necessary for these aspects. In order to design the signal it is therefore necessary to first determine the signal aspects that the individual signals must be able to display. As an example the following interlocking ("XQ") is used:
XQ is the point where a double track mainline (from left) diverges into two single track lines. The controlled signals are 10R, 12R, 10L and 12L, and both crossovers permit Medium speed. Approach signals are the automatic block signals 40, 37 and 17.
Signals 41, 36 and 16 are standard intermediate automatic block signals. As described in the "Signal basics" document, these signals can all display the "Clear", "Approach" and "Stop and proceed" aspects. As signals 36 and 16 are on bi-directionally signaled tracks, they are also equipped for displaying the "Stop" aspect, though the physical signals 36 and 16 are identical to signal 40:
Determining the aspects that can be displayed in the other signals requires deeper analyses, since the signaling depend on the track and signal layout in question. The factors that determine the signal aspects (permissible speed, block length, available signal aspects) has been explained deeper in "Basic Signaling", but without a tool to give an overview.
One tool to describe the individual signals is a signal aspect sequence chart. The chart shows all signals for a given direction of travel, their aspects and the interconnection between aspects in following signals that provide the advance signaling. The chart thus clearly lists the signal aspects of each signal and which signal aspect is displayed in which situation. The following is implicitly assumed:
All controlled signals display the "Stop" aspect if no route is set.
All automatic signals on bi-directionally signaled track display the "Stop" aspect if the direction of travel is opposing the signal.
The signal chart for XQ interlocking for "direction left" is shown below (symbols on chart are "home made" as I do not have a US prototype to work from):
A few less logical aspect sequences require some explanation:
Due to the lack of a "Medium approach" signal aspect, signal 12L displays "Slow approach" if signal 41 displays "Stop (and proceed)".
Signal 17 does not display "Approach slow" when 12L displays "Slow approach". This is commonly seen on PRR interlocking charts and seems to be a practice to save lamps in signal 17. For the train "Approach slow" and "Approach" yields nearly the same speed curve, and if visibility of signal 12L is good will not add to the running time.
Controlled signals display "Stop and Proceed" as their default aspect when route is cleared to a track with current of traffic away from the interlocking, and when route is cleared to bi-directional track (route can only be cleared if direction of traffic is away from the interlocking).
Routes can be set towards tracks with current of traffic towards the interlocking. Controlled signals display "Restricting" to these routes. Note that trains may not leave interlocking limits to such a track without a Train Order or other permission from the dispatcher/tower operator.
For "direction right" the signal aspect sequence chart looks like this:
The notes about "Slow approach"/"Approach" in signals 12L and 17 are also valid for signals 12R and 40.
As dwarf signals cannot display "Stop and proceed", signal 10R displays "Restricting" instead.
With the above in place, it is now possible to list all possible aspects in a given signal and thus determine its physical characteristics. The following illustrations show signals 10L, 12L and 12R and their approach signals (37, 17 resp. 40):
Similarly for dwarf signal 10R, though there's not approach signal:
Signal symbols on interlocking diagrams are derived from symbols for semaphore signals. High position light signals are shown as an upper and a lower arm, corresponding to the upper and lower signal "heads". Dwarf signals are shown as a one arm equivalent signal on a low mast:
The "arm" symbols symbolize the positions that the arm can take, i.e. the aspects that the signal can display. Please note that apart from the aspects shown, a lower arm can be extinguished, while the upper can not:
The arm symbols also show the normal position of the arm, as a bold symbol. Normal position is for all controlled signals "Stop", for all automatic signals normal position is the normal direction of travel for the line, with no trains on the line. The approach signals to interlockings thus normally show "Approach":
The signal symbols also shows the control method of the signal:
"Non-automatic" and "semi-automatic" are both controlled signals. Non-automatics requires the tower operator/dispatcher to clear the signal for each movement, while semi-automatic signals can be fleeted. It is common for fleeting only to apply to certain routes, usually the straight moves through the interlocking. Automatic signals are automatic block signals.
For dwarf signals and one-arm signals the diagram symbols shows all aspects that the signal can show. Two-arm signals could in theory show all combinations of the two arms, but these includes several aspects not defined in the rule book and which can be disregarded. Using the example two-arm signal from above, this yields the following combinations:
Additionally, the diagram symbols show that this is an automatic signal (i.e. not protecting a junction), further eliminating the "Slow approach" and "Medium clear" aspects:
With all the signal knowledge explained so far, it is now time to look at how to reverse engineer an interlocking diagram. Unfortunately this is somewhat an "art" but through examples it should be possible to cover enough cases to provide a solid foundation in this field. It shall be noted, that the above is based on a somewhat simplified PRR signaling concept, so real interlocking diagrams may show signal aspects and principles not covered here. In order to start gently, we'll study a fairly simple interlocking. Searching through Mark Bej's interlocking diagrams, Homewood PA was chosen.
Homewood PA is a junction where a single track branches off the double track (current of traffic) between Pittsburgh and Bycyrus. The interlocking diagram looks like this (non-compressed version for printing can be found here):
The interlocking diagram contains much information, of which not all is needed to reverse engineer the signaling. In order to de-clutter the diagram, the diagram below has the unnecessary information removed (printable large resolution diagram here):
Most numbering of signals and switches refers to the lever number in the interlocking frame of the lever that operates the object in question. Dwarf signal 26 is thus operated by lever 26. Note that two signals are operated by lever 25 - the operator lines the switches then requests a movement through them by operating lever 25. The interlocking will then, based on the switch positions, clear the correct signal.
From the de-cluttered diagram has been removed track circuit numbers, lever numbers for facing point locks, general information of the interlocking frame and revision history. The indication of a road bridge near signals 28 and 25c has also been removed.
Looking at the diagram, it is seen from the designations and unidirectional signaling that the main tracks are unidirectionally signaled. The branch, however, has the indication "Traffic Lever", indication bi-directional traffic. A traffic lever is a lever that locks the selected direction of travel on the single track. The bi-directional traffic is also indicated by the signals for both directions. Track 102 seems to be a secondary track not used for normal traffic. The wye track is not controlled by the interlocking as switches are not designated lever numbers and thus the wye track can be ignored when studying signal aspects. The wye track switch positions are of course still supervised by the signal system.
Signals 368, 358 and 331 are approach signals to the controlled signal at the interlocking. The diagram shows the signal spacing between approach and home signals to be "long enough" not to present special problems. Signal 367 and 346 are automatic block signals. Signal 367 is located far after the interlocking, while signal 346 is located right at the interlocking limits. The diagram shows controlled signal 1 and automatic signal 346 to be placed on a signal bridge spanning the two main tracks. Signal 24 is a semi-automatic signal, located at the interlocking limits. Other information from the interlocking diagram is that all switches are #10, i.e. slow speed only.
When studying an interlocking diagram it is also necessary to consider the operations (traffic patterns) that the interlocking must support. In the case of Homewood this is rather simple:
straight through on the main tracks
from signal 1 to the branch
from the branch to signal 346
secondary "restricting" routes as the track layout permits
From the signal 1 diagram symbol it can be seen that the signal in theory can display the "Clear", ""Approach slow", "Approach", "Slow approach", "Stop and proceed", "Restricting" and "Stop" aspects.
Looking at the straight route, the next following signal is signal 367, an automatic located "far away". With no diverging switches, signal aspects on this route will thus be the standard "Clear", "Approach" and "Stop and Proceed" aspects.
From signal 1 to the branch the next following signal is signal 24. The distance between these two signals is short, and besides switch 19 has to be traversed at Slow speed. The signal aspect will thus be "Slow approach". Signal 331 can be seen not being able to display "Approach slow", and will thus show "Approach" when signal 1 displays "Slow clear".
Signal 24 is located somewhat irregularly (at the exit of an interlocking) when considering the preceding signal principle description. But signal 1 can only display the "Slow approach" aspect to the branch (since the PRR signal aspects do not include a "Slow clear" from high signals), and thus a train will have to approach the next signal prepared to stop. As next signal would normally be "far away", this is an unacceptably slow procedure. By adding signal 24, trains will pick up a better signal indication when leaving the interlocking, allowing them to continue at line speed after clearing the interlocking. While traveling at Slow speed inside the interlocking it does not make any significant difference that the train on an "Approach" aspect.
Signal 1 will most likely display the "restricting" aspect if the route to signal 24 is occupied and always to track 102. Signal 24 will act as a normal automatic block signal once cleared.
Signal 6 is a "wrong main" entry signal to the interlocking, and since used rarely is can only display "Restricting" aspect for all routes.
Eastward movements from the branch to No. 1 track presents the same problem as westward movement to the branch. Signal 25a/b can thus only show "Slow approach", "Stop and proceed", Restricting" and "Stop". Signal 346 is therefore located at the exit of the interlocking, similarly to signal 24. Signal 358 can be seen not being able to display "Clear", since signal 25a/b cannot display aspects better than "Slow approach".
The distance between signal 28 and signal 346 is short, recalling the "short block" problem. Signal 28 can be seen only being capable of displaying the "Clear", "Slow clear", "Stop and Proceed" and Stop" aspects, so the solution used here has been to signals 368 and 28 display "Approach" and "Slow approach" respectively when signal 346 is at "Stop and proceed".
Signals 25c and 26 are similar to signal 6 in allowing unusual movements by the "Restricting" aspect.
Text, Images, HTML: Carsten S. Lundsten.
