Final answer:
Testing of the pre-emption of a crossing is required at least quarterly or when installed, modified, or disarranged.
Step-by-step explanation:
The question is related to the testing of the pre-emption of a crossing, which is a procedure used in engineering to ensure the proper functioning of crossing systems. Testing of the pre-emption of a crossing is required at least quarterly or when the crossing is installed, modified, or disarranged. Regular testing is necessary to identify any issues or malfunctions that may affect the safety and efficiency of the crossing system.
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