198007 – Cox – Track Circuits on Victorian System and Shunting Effectiveness

Although a track circuit if defined in the broad sense could include any track-side circuit used for train detection, the definition is taken to be, “a track circuit is one where the running rails are used for power distribution and the trains wheels act as a shunt for this power, over limits of length, to operate control devices and provide continuous indication of a train’s presence in the section”.

Systems such as presence detectors, wave guides, radar, directional transducers and magnetic and mechanical track devices do not comply with the above definition.

With the introduction of electric traction in the early part of this century, standard track circuit designs were produced and as these were both efficient and reliable for the conditions then applying there was no major incentive for further detailed research until the general program of modernisation of railway facilities immediately following the second world war.

Use of higher voltage A.C. traction increased the problems of circulating A.C. power in the rail and to permit longer circuits and more reliable operation, valve type generators and receivers operating at a frequency sufficiently removed from the traction fundamental frequency were developed for the French railways. This break through, in track circuit design together with the development of the transistor has led to audio frequency and jointless track circuits and equipment suitable for consolidation at a central location.

Other equipment has been designed in response to the needs for more efficient track side and cab signalling, automatic train operation, and improved level crossing protection. To overcome problems associated with technical advances in A.C. traction, static traction, generating and control devices, conducive to interfering signals in track circuits, long welded rails and the extension of signalling to light traffic density lines it has become necessary to make a complete re-appraisal of track circuits, and their operation in recent years.