From: joekor@earthlink.net   
      
   On 4/2/2016 8:51 AM, hounslow3@yahoo.co.uk wrote:   
   > On 01.04.16 21:22, Joseph D. Korman wrote:   
   >> That may be on the NEC, but it's not on the NY subway, that I know of. A   
   >> R/R home signal is stop and stay.  R/R/W is a call on.   
   >   
   > I thought RRW would be a one-shot signal.   
   >   
      
      
   No, that's R/W, a one-shot grade time control.  They are usually used at   
   curves rather than grade timers. The two shot timers are Y/S followed by   
   a single R.  If the train slows enough, the Y/S goes green and the R   
   goes to Y/S until the train exits the timed area.   
      
   If the train is too fast, the second signal remains at stop until the   
   train is under control.  If it doesn't slow down enough, it'll get tripped.   
      
   Grade timers may also be included in interlockings with a Y/D meaning   
   proceed at authorized speed with the next switch set for the divergent   
   route, protected by a R/R which would change to Y/Y if the speed is met.   
      
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