The July 1951 issue of Popular Science magazine described in detail   
   the cab signal and speed control systems available at the time.   
   One of the systems was the automatic speed control in use on   
   parts of the PRR and LIRR.  It was mandated on the LIRR after two   
   very deadly wrecks.  In that, an axle governor measures speed   
   and it is compared to the signal indication, if the speed is in   
   excess, the train is stopped.   
      
   I have no idea how these compares with the capabilities of modern   
   ACESES on the NEC, but that automatic speed control seems to cover   
   most situations.   
      
   I would think if it had been in place, it would've prevented the   
   recent Metro North and Amtrak NEC accidents were excessive speed   
   was the cause.   
      
   The article may be found at:   
   https://books.google.com/books?id=0SADAAAAMBAJ&lpg=PA52&dq=Penns   
   lvania%20railroad%20cab%20signal&pg=PA50#v=onepage&q=Pennsylvani   
   %20railroad%20cab%20signal&f=false   
      
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