I’ve been giving your question a lot of thought MattW regarding retention of ABS/TWC from Scottsdale to Lithonia. It’s important to remember that the function of ABS in Dark Territory is added capacity not necessarily added safety. If TWC is used properly there should be no problems. Without supplementation of ABS, trains would have to proceed at slower speeds in Dark Territory, mandated by the FRA. Remember that the line between Scottsdale and Greensboro is APB (Absolute Permissive Block) ‘Poor Man’s CTC’, a sophisticated form of ABS. In APB the signals at sidings often have a top hat and lower hat, the top to protect through movements the lower to protect sidings. Trains in APB must come to a complete stop at ‘absolute’ signals just as they do in CTC; they cannot proceed under stop as in ABS. There are six signals between Scottsdale and Lithonia, three standard ‘permissive’ block at Clarkston, Goldsmith road and near Redan; three ‘absolute’ signals at Scottsdale, Stone Mountain 154.54, and Lithonia 153.0 (you can identify an absolute signal in ABS territory by the absence of number plating on the signal). 153.0 represents a unique situation for absolute stop because of the ‘G’ label, indicating although it’s absolute, trains may proceed at restricted speed because if they were to stop they would have a difficult time climbing the hill (‘G’ means Grade).
As you so pointed out the line is largely single tracked from Scottsdale to Stone Mountain limiting the number of trains CSX can run in bi-directional operation. Originally there were ten pairs of signals from Scottsdale to Lithonia for normal ABS controlled by telephone pole wires and track circuits when trains ran more frequently than they do now, even after DTC replaced manual block rules enforced by station masters. CSX never buried cable for the Georgia line but instead installed a pulse coded rail signaling system to replace telephone poles from Clarkston to Augusta. Regardless, quite a few local trains still travel back and forth from Lithonia to Avondale and Atlanta weekly in addition to long haul freight. Because it’s APB the Scottdale signal protects the NB Stone Mountain siding, the main function of APB being to protect movements on single track between sidings. Whereas the original eastbound US&S searchlight at Scottsdale marked the end of TCS territory control, the replacement will continue to do so with its top Hat while the new lower Hat no doubt will be used for PTC protocol to Stone Mountain in conjunction with the top Hat (the line from Scottsdale to Greensboro was converted to APB after TCS was installed from Scottsdale to Kirkwood).
The 154.54 SB Stone Mountain never had a lower Hat until the new plans for PTC, possibly to insure protection of the advance approach signal at Redan and the daily traffic on the Lithonia Industrial spur that supports several customers. When Track Warrant is in effect (or, Work & Time Authority) for locals to work the spur quite often the switch machine on the mainline is fouled so that train crews can serve customers as long as three hours. This is especially true of local traffic east of Lithonia all the way to Madison. I think that APB will remain in effect supplemented by PTC, the switch over happening when needed.
The hyperlink for GE Electro-Code, a standard for pulse coded rail signaling system;
http://www.getransportation.com/its/sig ... -interface" onclick="window.open(this.href);return false;
The hyperlink for information on APB;
http://www.lundsten.dk/us_signaling/abs_apb/" onclick="window.open(this.href);return false;