What must not be left unattended during subway flagging procedures?

During subway flagging procedures, it is critical to ensure that the lights in a flagging arrangement are not left unattended. This is because these lights serve as crucial signals for train operators and other personnel, indicating the status of the work zone and the safety measures in place. Failure to monitor or maintain these lights could lead to confusion or miscommunication, potentially resulting in unsafe conditions for both workers and train operators navigating the area.

In contrast, training manuals, stationary trains, and conductor's tools, while important in their own contexts, do not play an immediate role in signaling safety during flagging operations. Training manuals can be reviewed and referred to as needed, stationary trains, being secured, are typically not a hazard, and conductor's tools are often under the direct supervision of the conductor or personnel using them. However, the lights are vital for immediate communication regarding safety and must be actively observed to ensure they function correctly and convey the necessary information to those involved in the flagging operation.