of the signal, most railroads elect to set intermediate Signal 246 to “Approach” rather than implementing an “Approach Restricting.” In our example, the differ- ence would be that “Approach Restricting” would require the engineer of Train A to approach Signal 4R at not more than “Re- stricted Speed” versus when Intermediate Signal 246 is at “Approach,” the engineer would be required to approach Signal 4R prepared to stop. It is always good, as done on the proto-
type, to seek out heads that do not need to be controled, i.e. they can have a fixed red aspect. For example let’s assume in Fig- ure 1f that instead of routing a train into the yard, a train is being routed out of the yard. What are the possible aspects for Sig- nal 4LC? Because Turnout 5 is a number 12 prototype and 6 model, the only pos- sible aspects are “Slow Clear” if Signal 245 is not red and “Slow Approach”, if Signal 245 is red. Looking at Figure 2, the cor- responding aspects are red-over-green and red-over-flashing yellow respectively. Thus the upper head can be a fixed red wired permanently with no need to connect to a C/MRI output line. Using the aspects de- fined in Figure 3 we would have the same result but with green-over-red and yellow- over-red but In this case the lower head would have the fixed red. Although it is not the case with the majority of fixed aspect heads, in this case the upper head can be eliminated entirely using a solid green or flashing yellow for the single head indepen- dent of whether using Figure 2 or 3 denot- ed aspects. Some prototypes provide even more
aspects than are listed in Figure 2. An ex- ample is “Medium Approach Slow,” whose meaning should now be somewhat obvi- ous: Proceed at “Medium Speed” (30 mph) within interlocking limits then proceed ap- proaching next signal at “Slow Speed.” To clarify this point, when you encoun-
ter multiple “speed designation words” within a signal’s name, the first one defines the speed at which a train may pass that particular signal, and the second defines the speed at which the train may pass the next signal in advance. For example, a sig- nal displaying “Medium Approach Slow” simply states, pass that particular signal at medium speed and approach the next sig- nal in advance at slow speed.
Figure 3
Clear
Restricting
Limited Clear
Limited Approach
Approach Limited
Medium Clear
Medium Approach
Approach Medium
Stop
Slow Clear
Slow Approach
Approach Slow
Increasing the number of aspects displayable with dwarf (pot) sig- nals. These aspects are per NORAC operati ng rules used by Conrail and many other northeastern railroads.
Typically with earlier and more stringent speed signaling applicati ons, 3-headed mast signals are located at both ends of passing sidings for displaying medium clear, limited clear, or slow clear, depen- dent upon the frog angle of the turnout. This is the approach illustrated in Figures 1a through 1c. Oft en to fi t in the mast signal requires widening the track spac- ing, e.g. adding a dog leg, or placing the signals on a canti lever. To circumvent these added expenses, a goodly number of speed signaling railroads elect to ap- ply dwarf (or pot) signals at each end of a passing siding much like is typical with route signaling. However, with speed signaling many more aspects are need- ed which are att ainable by incorporati ng fl ashing aspects (see Figure 3). Where diff erent, these aspects replace the dwarf signal aspects defi ned in Figure 2. Additi onally, for illustrati on purposes we have used the Figure 3 aspects in the Figures 1d through 1f examples.
With speed signaling, as implemented
Important factor in helping to understand signal Names and Indications
In general, with speed signaling, you can
think of every signal aspect providing two essential elements of information through its aspects and associated indications. First is the speed at which a train may pass that particular signal, and second is the speed at which the train may pass the next signal in advance or action that must be taken at the next signal. This same thought process holds for al-
most every signal name, except that when the next signal in advance indicates a stop, i.e,. zero speed, the term “approach” is used by itself rather than saying “approach stop,” and the absence of a word prior to “approach” indicates that the train may pass that particular signal at not exceeding maximum authorized track speed. Thus, Approach indicates passing that particu- lar signal at track speed while approach- ing next signal prepared to stop, Approach Medium indicates passing that particular signal at track speed approaching next sig- nal at Medium Speed, and so forth.
using Figure 2, you typically need three- head signals leading into the point end of dispatcher- or tower operator-controlled turnouts. In our modeling case, it is pos- sible to reduce these to only two-head sig- nals; however, in doing so, you eliminate the Medium Clear, Limited Clear, Medi- um Approach, and Limited Approach. As such, every movement through every di- verging route would need to be taken at slow speed (20 mph) using either the Slow Clear or Slow Approach. For many applica- tions that is not much of a handicap. For example, if your sidings are limited to less than 30 mph, which is typical on many prototypes, there is little if any need for the faster speed diverging route aspects. However, the more complete implemen-
tation requires adding the third head for directing train crews to maintain Medi- um and Limited Speed through diverging routes. Fortunately, by using the C/MRI, all the more complex signal logic required for the added aspects is easily handled by soft- ware, which typically is a zero-cost item. Therefore, the added cost associated with implementing more complete speed signal- ing is the cost associated with adding a few more signal heads and associated I/O lines.
FEBRUARY 2016 59
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