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[quote user="Railway Man"] Fail-safe does not mean "never fails."  It means that certain failure modes are designed to fail to the safest possible condition.  Some common failure modes for grade-crossing signals that do not fail to a safe condition are: <OL> <LI>Vehicle runs over the mast and knocks it over.  It does not re-erect itself. <LI>Utility power fails, and for various reasons no one is available to repair the utility power or recharge the batteries when they poop out a few days later.  Think "Katrina."  <LI>Lightning strikes the instrument house directly.  The instrument house has lightning arrestors to prevent high-voltage transients from strikes to the power lines or nearby ground from entering the house, but they will be overwhelmed by a direct hit. <LI>Vehicle runs through the gates and breaks them off.  This happens so frequently that every signal-maintainer's truck carries a spare gate or two on the rack. <LI>Rail vehicle does not shunt the track.  A major problem with DMUs after a weekend when they only run on weekdays and it rains, and special measures have to be taken such as a light grind of the rail head early every Monday morning.</LI></OL> The most useful fail-safe mode is the shunt-to-activate mode.  That means, the signal system is designed so that any detectable occupancy of the track -- even a false occupancy -- activates the signal.  The relays are arranged so that they need to be "picked up" -- against gravity to turn the signal system off.  Thus, the relays are "normally on".  And so forth.  Any place where there is a reasonable and effective way to make the system fail to the safe mode, it's done.  But as listed above, not all failure modes can be made safe. RWM [/quote] RWM,  <?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" /><o:p></o:p> Thank you for that insight.  That is more less what I had concluded because I cannot see how any system could be perfectly infallible.  So it would seem that “failsafe” is sort of a term of art that can apply to some individual components of a system, but may not apply to the whole system.  One reference that I found describes the failsafe principle as being a design using the closed circuit principle.  I believe that this is what you refer to as a “shunt to activate” system.  But then the reference mentions the problem of there being no way to use a closed circuit design in the warning system without going to an open circuit design in the final stage that activates the signals.  In other words, a power failure drops the relay, but the contacts of the switch controlled by the relay could fail to close the activation circuit due to a number of possible causes.      <o:p></o:p> But here is why I asked the question: <o:p></o:p> During this thread, others had mentioned the driver’s responsibility to yield by slowing down, looking for trains, and stopping if necessary to avoid being hit by a train that might be approaching.  That is the meaning of yield, and the crossbuck is equivalent to a YIELD sign.  While most people are aware of this responsibility at passive or non-signalized crossings, what was claimed by others in this thread is that the same responsibility applies at signalized crossings when the signals are un-activated.  This is because signalized crossings also have a crossbuck, and its meaning prevails whether the signals are activated or not.   <o:p></o:p> I sought confirmation of the premise (that divers must yield at signalized crossings when the signals are un-activated) to Operation Lifesaver and the FRA, and they told me this:  “The crossbuck sign means yield. It is the motorist responsibility to slow down, look in both directions and determine if a train is approaching. The train always has the right of way. This is required if the grade crossing is equipped with active warning devices or only the crossbuck.”<o:p></o:p> <o:p></o:p>   The only point that is missing is whether the yielding at a crossing with warning signals is necessary if the signals are un-activated.  Logically, it would be necessary because the crossbuck is there with its yield command no matter whether the signals are activated or not.   <o:p></o:p> I was sent a publication showing the rules, signs, and signals at grade crossings.  It clearly spelled out the meaning of the crossbuck at non-signalized crossings.  However, it did not even mention the crossbuck at signalized crossings.  Nor did it mention a need to yield at signalized crossings when the signals are un-activated. <o:p></o:p> I contacted a representative from the State Patrol about this question.  He said that he did not know how the law would apply to the situation, but he seemed skeptical that there was a legal requirement to yield at a signalized crossing when the signals are un-activated.   <o:p></o:p> I have learned of a recent comprehensive study that has concluded that motorists do not recognize the crossbuck as a YIELD sign.  Therefore, there are plans to add YIELD signs to the crossbucks at non-signalized crossings.  Based on this development, I ask this question: <o:p></o:p> If there is a need to add yield signs to passive crossings because much of the public does not understand that the crossbuck means yield, then why is there not the same need to add yield signs to signalized crossings?   <o:p></o:p> I posed this question to the state D.O.T. and was told that there is no need to advise a motorist to yield/stop at a grade crossing with active warning devices because those warning systems are required to be failsafe.  Basically, it was explained to me that the failsafe attribute meant that the gates and signals cannot fail to activate.  I was told that adding a YIELD sign to a signalized crossing would not only be unnecessary, but it would also confuse motorists.  <o:p></o:p> This response raises these issues: <o:p></o:p> 1)          If it is true, there is no need for crossbucks at signalized crossings. <o:p></o:p> 2)          It cannot be true because failsafe does not mean that the signals cannot fail. <o:p></o:p> And it raises this difficult question: <o:p></o:p> Why would a YIELD sign added to the crossbuck confuse drivers at a signalized crossing, but add clarity at a non-signalized crossing? <o:p></o:p>
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