Which of the following indicates the limits of an Automatic Block System?

The limits of an Automatic Block System are indicated by the positions of block signals. These signals are critical components of the Automatic Block System as they govern train movements and provide information regarding track occupancy. Block signals are strategically placed along the track to specify whether the sections ahead are clear for train passage or if they are occupied or restricted.

When a block signal displays a clear indication, it signifies that the track section is safe for a train to proceed, while a stop indication warns the train operator to halt due to either an occupied block or an upcoming restriction. The spacing and placement of these signals are designed to ensure efficient and safe train operations, enabling automatic control over train movements based on real-time track conditions.

In contrast, the length of a train, manual control of train operators, and the type of cargo being transported are not relevant indicators for the limits of an Automatic Block System, as they do not directly relate to the signaling system's function or design. The operation and safety of the block system rely entirely on the proper functioning of the signals, making them the definitive indicator of the system's operational limits.