What does a tonnage signal inform train crew about?

A tonnage signal provides essential information regarding the loaded status of a train, specifically indicating the requirements for how much weight the train can safely transport on a given line. This signal helps train crews understand the constraints and operational capacities related to the weight they are handling, which is crucial for safe and efficient train operations.

Recognizing the tonnage allows the crew to adjust their handling of the train, ensuring that they adhere to any speed restrictions or braking requirements associated with the weight being carried. This is vitally important for maintaining safety standards and for preventing incidents that could arise from overstressing track infrastructure or not being able to stop the train effectively due to its load.

The other choices, while relevant in their contexts, do not directly relate to the function of a tonnage signal. Track conditions ahead, weather changes, and signal malfunctions are important aspects of train operation but are informed through different signals or communication, rather than a tonnage signal specifically.