What is a cab signal primarily used for?

A cab signal is primarily utilized to indicate the permissible speed to the engineer. This system is crucial for maintaining the safe operation of trains, especially in areas where track conditions or signals may change rapidly. By providing real-time information directly in the locomotive cab, the engineer can adjust the speed of the train accordingly to ensure it remains within safe operating limits.

This function enhances safety by allowing the engineer to react to conditions on the track without having to rely solely on wayside signals, which may not always be in their line of sight or could be obscured. Understanding the permissible speed allows for better management of braking distances and overall train operation, contributing to accident prevention and long-term safety in rail transport.

The other options listed do not align with the primary function of cab signals, which is specifically to convey speed-related information rather than weather updates, communication with other trains, or synchronizing train movements.