going through the island such as a pso signal a 44 does not drop on this train move due to the train not entering the block or b pso signal a 44 drops but the train leaves the crossing approach via a switch or the train backs out thus to solve situation a a 10 second timer is started after pso signal b 45 picks this is seen in block 56 of fig2 if pso signal a 44 does not drop in this time bypass is not allowed however if signal a 44 drops bypass occurs and the uax input to controller 40 remains energized block 58 in fig2 in order to solve situation b the bypass is ended if the ez level goes above 80 however a timer is needed to prevent the bypass being canceled as the train first enters the approach hence a 60 second timer is started when pso signal a 44 first drops block 58 in fig2 when the 60 second timer expires block 60 the remote remains bypassed and the uax input to controller 40 remains energized if signal a 44 returns or if the ez level is greater than 80 for about 5 seconds the process returns to the idle block 52 in fig2 as can be appreciated if a more sophisticated pso transmitter is used that can convey more than 1 bit of information for instance being able to dynamically switch between transmitting one or two code signatures and if the pso receiver can distinguish between these codes then a single pso transmitter could be used instead of a separate pso transmitter for signal a 44 and a separate.