SENDING OUT THE RIGHT SIGNALS

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The safe working of any rail network is critically dependent on effective train signalling, protection and control systems. Traditionally each route is divided into fixed length sections, with movement controlled by lineside signals and train detection equipment locating the position of trains within these sections. The most important advantage of this fixed block system is that only one train can be on the same section of track at any one instant. All signalling systems are designed to indicate to the train crew that it is safe to proceed, brake, or stop. Getting these messages to the crew, as well as knowing where each train is on any section of line has changed significantly over the years.   Now, a new generation of transmission and communications based signalling technology is likely, ultimately, to replace existing lineside hardware, and electro-mechanical systems.

Basic Signalling Technology

One of the key features of the technology is the interlocking of switches and crossings with lineside signalling equipment, using firstly all mechanical and later electro-mechanical techniques.

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The provision of Automatic Train Stop (ATS) equipment, has been used by all Japanese railways following two serious rail accidents in the early 1960s. ATS provides an audible warning in the cab as the train approaches a stop signal, if the driver does not apply the brakes within 5 seconds, the emergency brakes are automatically applied. In a recent extension of this system ground based coils installed along the track monitor the train braking patterns, and if permitted speeds are exceeded, the system automatically applies the service brake.

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Solid State Interlocking (SSI)

Interlocking switches and crossings with the displays from lineside signals is a key part of route setting and train movement control, but the electro-mechanical systems of the past have given way to electronic, computer-controlled systems. Electronic or solid state interlocking to control route setting and signal displays covers three areas:

  • The combination of hardware and software to maintain the logic for safe operation of trains – the control centre equipment.
  • The medium – copper or fibre-optic cable – connecting the train control centre equipment to the trackside equipment.
  • The interface between the trackside and control centre based equipment.

Solid State Interlocking carries out the functions of traditional electro-mechanical systems using computers and software.

Moving Block – Transmission Based Signalling

Improvements in track to train communications have been a driving force behind the success of transmission based signalling systems. The detailed information about speed, braking distances, headway and even the status of the train’s on-board, power or control systems cannot be achieved by conventional lineside signalling alone.

Moving block or transmission based signalling provides the capability to maintain closer headway between trains. In turn, providing denser traffic flows, and of course, to the train operator, more trains per line means greater revenue.

IMAGE02 - Moving Block Principle

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This feature originally published in “Asia Pacific Rail” magazine; January 2002

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