What characterizes an Automatic Block Signal System (ABS)?

An Automatic Block Signal System (ABS) is characterized by a series of consecutive blocks that are governed by specific block signals and the conditions of the track. Each block is a section of track that is protected by signals, which convey the status of the block ahead to train crews. This system ensures safe spacing between trains, automatically controlling train movements and reducing the risk of collisions.

By providing signals based on track occupancy, the ABS enables efficient management of train movements, allowing for automatic control of signals as trains enter and exit blocks. This not only enhances safety but also improves the overall flow of rail traffic, as trains can be spaced appropriately based on real-time conditions.

The other options do not accurately reflect the nature of ABS. It is not limited to freight trains, as it can be used for passenger trains as well. It does not function as a scheduling system but rather focuses on real-time control and safety. Additionally, while manual signaling may be used during emergencies, ABS itself is designed to operate automatically, without the need for manual intervention under normal circumstances.