wrote:   
      
   > As you and others have pointed out, it is the combination of combustible   
   > vapors and oxygen that is particularly dangerous. At one time, it was   
   > quite common for ocean going crude oil tankers to explode at sea due to   
   > the volatile fractions of the crude oil combining with the air in the   
   > cargo tanks to form an explosive mixture. This problem has largely gone   
   > away since tankers have been required to fill the "ullage" space (the   
   > space above the liquid in the cargo tanks) with filtered exhaust gas(i.e.   
   > nitrogen and carbon dioxide) from the engines so as to exclude oxygen.   
   > Similarly, new design airliners are to be required to fill the ullage   
   > space in fuselage tanks with inert gas. Is it practical to apply this to   
   rail tank cars?   
   >   
      
   So far, all of the fires involving flammable liquids in tank cars are   
   believed to have happened after the vessel leaked or ruptured, so inert   
   gases in the tank wouldn't seem to provide much protection.   
      
   Given that Bakken crude has a low specific gravity, I think that tank cars   
   transporting it are probably pretty much full anyways (i.e. they run out of   
   volume before reaching the max allowable weight). There would be limited   
   oxygen available to feed a fire inside a non-compromised tank.   
      
   Dan   
      
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