From: heller@deepsoft.com   
      
   At Tue, 5 Apr 2016 11:53:19 -0700 (PDT) hancock4@bbs.cpcn.com wrote:   
      
   >   
   > On Monday, April 4, 2016 at 10:24:23 PM UTC-4, John Albert wrote:   
   >   
   > > Hard to say at this point.   
   >   
   > In the previous very similar accident about 20 years ago, they   
   > said MOW vehicles were purposely set up NOT to shunt track   
   > circuits.  This was because such units didn't always shunt, so   
   > it was felt it was best to assume they didn't.  Thus, such a vehicle   
   > wouldn't trip the signals.   
   >   
   > Is this still true?   
   >   
   > It seems to me they should've gone the opposite way--put something   
   > on the vehicles to ensure they do shunt the signal circuits and   
   > create occupancy.   
      
   I suspect it is a matter of weight.  Steel/iron, while certainly conductive,   
   is hard to make a really good electrical contact with (and rust is an   
   insulator).  Locos and most rail cars are very heavy and press really hard   
   against the rails (making a good electrical contact).  MOW equipment is   
   *mostly* much lighter and are just not going to make a good contact.  The only   
   way to 'ensure' that MOW equipment make a really good contact is to add *vast*   
   amounts of weight to them.  Which in some cases would be counterproductive.   
      
   I guess, some sort a 'jumper cable' could be used to 'fake' occupency.   
      
   >   
   > (ABC News got it wrong--they said the signals check for speed   
   > but not obstructions.)   
   >   
      
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