What is essential for the validity of the "Slow-By" restrictions?

The validity of "Slow-By" restrictions hinges on the presence of accompanying Speed Signs. These signs serve as vital visual cues that inform train operators of the necessary speed reductions at specific locations. Without these signs to alert and guide engineers, the execution of the "Slow-By" restrictions would lack the necessary enforcement mechanism, leading to potential safety risks. The signs are critical because they ensure that operators are aware of and can comply with the required changes in speed in real-time while operating the train.

Documentation in the Train Movement Office, publication in bulletins, or the requirement for these restrictions to be in effect at all times are important aspects of train operations and safety management, but they do not directly reinforce the validity of "Slow-By" restrictions as effectively as the placement of Speed Signs does. The signs provide immediate and clear communication to train crews, which is why their presence is essential for ensuring that "Slow-By" restrictions are adhered to.