What does one long blast on the tower whistle signal to all trains within interlocking limits?

One long blast on the tower whistle is a critical communication signal used in railway operations, especially within interlocking limits. This specific signal indicates to all trains in the vicinity that they must come to an immediate stop. The reason this signal demands such an urgent action is due to the potential for imminent safety hazards, such as conflicting train movements, maintenance activities on the track, or other urgent conditions that could affect the routing and movement of trains.

Understanding the purpose of the long blast is essential for train operators as it ensures safety and coordination among rail traffic. This practice helps prevent collisions and accidents by effectively communicating the need for all trains to halt until they receive further information or instructions from the control tower or signal system.