This elaborate system is essential to ensure that the correct messages are received and understood by the designated crew. A summary of how the system works is as follows. An unsignalled line is broken up into blocks which can be up to 20 miles long. A BLS consists of a simple fixed sign with the name of the BLS on it. Block Limit Stations are usually located away from sidings so that any shunting can be done within the limits of a single block.
Train operation is controlled by a dispatcher.
This will be his authority to proceed. The Form D tells the train which blocks it can be in. In this way the dispatcher knows which blocks are occupied and, therefore, where trains can run safely. The section at the top has automatic block signals and remote interlockings while the bottom is DCS controlled. An alternative description of the use of track warrants is at a useful page called Track Warrant Control .