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Modern communications based signalling places improved signalling functionality on board the train.

This can be used to enforce conventional temporary speed restrictions using location based authorities. With these the train ensures its speed is maintained below the temporary maximum between two defined points.

In a related class are time based authorities. A time based authority commences at a specified time and continue to a specified event (which is not necessarily time based). Two examples are presented.

The first relates to a requirement to restrict passing speeds within a long tunnel to below a specified maximum (as is the case for the Seikan tunnel in northern Japan).

In this case the signalling system is aware of the location and authorized speed of the two passing trains in advance. With this knowledge a passing point can be predicted in terms of location. However, a speed restriction based on this criterion can be shown to be unsound as a provider of safety. Thus a safety benefit is obtained by defining the passing point in terms of time; a time based authority emerges.

The second relates to level crossing protection.

It is conventional in a class of signalling to require a train to obtain an authority to cross a protected level crossing.

Communications base signalling allows a train to communicate its arrival time to the level crossing as part of the process for obtaining that authority. This is another class of time based authority – the train obtains authority to cross at a specified time.

Once communicated, the train is able to regulate its progress safely to ensure it does not arrive prior to the specified time. The crossing is able to ensure that the standard warning is provided prior to the authorised arrival time.

The paper explores the characteristics of, and requirements for time based authorities.