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Monday, March 17, 2014

Better Know a Signaling System - Southern

The Southern Serves the South and today it is serving my need for a blog post.The Southern is interesting as it is the most Western of the Eastern Railroads in terms of its signal indications, yet still retains a number of very Eastern elements.  Until recently the South was a fairly laid back rail environment compared to the corridor running between the Northeast and Chicago.  With large amounts of single track ABS and little need for interlockings away from major junctions and station terminals the Southern was able to get by with a total of 10 signal indications vs 21 under NORAC and similar numbers on the CSX systems.

As you can see the Southern was a route signaled system which means that all the very specific Speed based signals just get tossed right into the bin and replaced with "Prepare o Diverge", "Diverge to Clear" and "Diverge to Stop".  In what might be a new level of laziness Rule 304 "Diverging Clear" actually informs crews that if an intermediate signal does not exist between there and the next interlocking they must approach that signal prepared to diverge as well.  So at that moment the engineer must be cognizant of the current turnout speed, if there is an intermediate signal and if not then the next turnout speed as well as any additional speed restrictions which are all recalled from memory.

I might not like Y/Y Advance Approach, but its better than Approach Diverging
 What makes the Southern stand out compared to the traditional Route Signaled lined out west is their use of Y/G for Approach Diverging as opposed to Y/Y on  the UP/BNSF systems, instead using Y/Y for Advance Approach instead of *Y*.  With Y/G a major aspect the system doesn't suffer from yellow overload.

There's something about green and yellow that just goes so well together.

Another defining feature of the Southern system is the use of bottom yellow Restricting.  R/Y won't cut it as that is Diverging Approach.  You need either a third head (the preferred method now employed by NS) or a sufficiently large "gap" between the top head and the one below it. Again this requires a bit of route knowledge to prevent mixups.  The Southern tended to employ Restricting sparingly, like for movements into yards.  Today NS uses it much more frequently.

Old school Restricting capable signal...mind the gap.

New NS signals always include the middle head.
Now unlike Conrail, NS has made no attempt to either mash all of its legacy signaling signals into one unified whole or phase the older styles out.  Therefore NS maintains separate signal charts for its Southern. N&W and NORAC zones.  Before we go any further lets see how much as changed.

I'll make things easier and point out that the only change is to the *Y* indication going from Approach Slow to Approach Restricting.  This gives us the opportunity to talk about why the route signaled Southern has this big old speed signal indication in it.  Rules 306 and 306.1 were added when flashing relays became available, sort of like Medium Approach on the PRR.  They were needed for the Southern's CTC projects that looked to replace the single track bi-directional ABS lines with manual passing sidings. The new CTC sidings were not signaled and thus the need for a new indication was born.  By flashing the yellow the existing ABS signals would not need physical modification with extra heads.  At some point NS changed the indication from Approach Slow to Approach Restricting since that is the indication the train would be approaching.

Another indication that seems to have bitten the dust between 1980 and 1981 is Rule 305 or Diverging Route Clear, Approach Limited.  Again this seems like one of those CTC related rules that was trying to fix the problem of back to back interlockings I mentioned before.  In this case the Southern used the old Limited Speed Triangle to tell engineers they were diverging into a single block siding and should expect to diverge again.  It appears the experiment was a failure as like I said no mention appears in the 1981 rulebook.

Moving on to dwarf signals R/Y for Diverging Approach is carried over leaving Y as the main Restricting indication.  Fortunately the use of virtual signal heads means that a three lamp dwarf with the order R-Y-G can display R/Y Diverging Approach or Y Restricting.  Like NORAC a 4th lamp is needed to display both Clear and Diverging Clear, but instead of G/G, the Southern employs G/R like the Western Roads. All indications have their own dwarf equivalent and moreover with the exception of Restricting all indications' dwarf signals mirror at least one high signals aspect. 

Even if one of the lamps is clearly more of an afterthought.

  Well with only 10 signal rules to cover that's pretty much it. Tune in next time for whatever it is I can find photos of.

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