The correct answer is: A. T/B 912
T/B 912 is a train order authority that allows trains to proceed through a section of track where the signals are not working. This authority is issued by the dispatcher and must be carried by the train crew.
T/B 912 is a temporary authority and is only valid for a specific period of time. Once the signals are repaired or the problem is resolved, the authority will be canceled.
T/B 912 is a safety measure that is designed to prevent accidents. By allowing trains to proceed through a section of track where the signals are not working, T/B 912 helps to ensure that trains can continue to operate safely.
The other options are incorrect because they do not authorize trains to proceed through a section of track where the signals are not working.
A. T/A 912 is a train order authority that allows trains to proceed through a section of track where the signals are working.
B. T/C 912 is a train order authority that allows trains to proceed through a section of track where the signals are working and the speed limit is reduced.
C. T/D 912 is a train order authority that allows trains to proceed through a section of track where the signals are working and the speed limit is increased.