From: stephen@sprunk.org   
      
   On 24-Dec-15 11:19, Adam H. Kerman wrote:   
   > Jishnu Mukerji  wrote:   
   >> The purpose is to indicate that Rule 281a must be followed, as   
   >> opposed to Rule 281 for a non-flashing green.   
   >   
   > I don't have NORAC signal rules handy. Looking for something on the   
   > Web, I came up with a scanned page with Rule 281 and Rule 281b, but   
   > no Rule 281a.   
   >   
   > You're making it sound like the wayside signal is instructing the   
   > engineer to obey the speed restriction displayed by the cab signal.   
   > Doesn't the wayside signal have the ability to convey the necessary   
   > information?   
      
   No; several new indications are not available in wayside signals, such   
   as Clear 125mph (used in areas where MAS is 135/150mph), Clear 100mph,   
   Cab Speed 80mph, and Cab Speed 60mph.  IOW, everything between Clear   
   (MAS) and Approach Medium (45mph).   
      
   Note that trains with only 4-aspect PRR cab signals will receive Cab   
   Speed 80mph as Approach Medium (45mph) and Clear 125mph as Clear (MAS),   
   and will not receive Cab Speed 60mph and Clear 100mph at all.  They also   
   can't distinguish between Restricting (20mph) and Stop.   
      
   S   
      
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