The correct answer is: A. T-511.
T-511 is a single line working instruction that allows for the use of a single track for the movement of trains while the other track is being worked on. This type of instruction is used when it is necessary to keep one track open for traffic while repairs or maintenance are being carried out on the other track.
T-511 instructions are issued by the railway company’s operations department and must be followed by all train crews. The instructions include information on the location of the work, the type of work being carried out, the speed restrictions that apply, and the signals that will be used to control train movements.
T-511 instructions are only used when it is absolutely necessary to keep one track open for traffic. When possible, all work is carried out on a closed line. However, there are times when this is not possible, such as when there is a major incident or when there is a need to carry out urgent repairs.
When T-511 instructions are used, it is important to ensure that all train crews are aware of the situation and that they are following the instructions correctly. This is to ensure the safety of both the train crews and the workers who are carrying out the work.
The other options are incorrect because they are not single line working instructions. A simple memo is a document that is used to communicate information between two or more people. T-512 is a single line working instruction that allows for the use of a single track for the movement of trains while the other track is being worked on, but it is only used in certain circumstances. T/D 602 is a document that is used to record the results of a track inspection.