What is the best immediate action if a train operator sees a broken rail on an adjacent track?

Applying the brakes in an emergency is indeed the best immediate action if a train operator sees a broken rail on an adjacent track. This action is crucial because it can help prevent an accident or derailment that might occur if the train continues moving at its current speed. The immediate response to a dangerous situation like a broken rail is to prioritize the safety of the train and its passengers, and quickly reducing speed is vital in mitigating the risk of a serious incident.

While notifying the conductor, sounding the horn, and considering a route change are also important parts of the safety protocol, they are not considered the best immediate response in this scenario. Notifying the conductor enables further communication and coordination, but it is not an action that will immediately prevent an accident. Sounding the horn might alert nearby workers or trains about the situation, but again, it doesn't directly address the immediate danger posed by the broken rail. Changing the route could be a necessary step thereafter, but it cannot be done instantaneously and does not provide an immediate safety measure in the moment the operator spots the broken rail. Thus, the most effective immediate action is to apply the brakes to ensure safety.