What aspect should be closely monitored for safety during train operation?

Monitoring signal integrity is vital for ensuring safety during train operation because signals are fundamental to the communication between the train and the control system. They provide essential information about track conditions, speed limits, and other critical operational data necessary for safe train movement. Signal integrity ensures that this information is accurate and reliable, allowing train operators to make informed decisions and maintain safe separation from other trains. Any failure or malfunction in the signaling system can lead to dangerous situations such as collisions or derailments, highlighting the imperative nature of maintaining signal integrity as a top safety priority in train operations.

While passenger behavior, weather conditions, and train schedule adherence are also important considerations for overall operational safety and efficiency, they do not have the direct and critical impact on the immediate safety of train movements like signal integrity does.