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2015 - October - Palazzi - Railway Capacity - Signalling amongst other influe 2015 - October - Palazzi - Railway Capacity - Signalling amongst other influences

Bill Palazzi B.Eng (Elec.) MIRSE


The layout and configuration of a signalling is a key factor in defining the capacity of a railway. However, the signalling system is not the only factor influencing capacity, and in fact many of the other issues can compromise the capacity delivered by the signalling system.

Capacity on any given infrastructure is partially about what is designed, but is also about how it is operated and what external influences there are. In this way, a railway is less like a measuring tape which provides a consistent and repeatable outcome, but is more like a tool where the quality of the outcome can be poor, acceptable or outstanding depending on the skill of the craftsperson.

To assist the understanding of railway capacity, this paper has outlined a hierarchy of influences on capacity which progressively constrain what is achievable in operation. The hierarchy incudes four levels of influence, as below:

  • Tier 1 Influences - Inherent factors; baseline infrastructure configuration
  • Tier 2 Influences - Design factors; signalling theoretical capacity
  • Tier 3 Influences - Achievable capacity: what can be timetabled
  • Tier 4 Influences - Delivered capacity; day of operation impacts.

The four tiers of influence help define how the various elements that make up capacity relate to each other, including the relationship between the signalling design and other influences. The tiers also help to clarify where signalling can help, but also the areas where signalling has little or no influence.

Finally, whilst optimising train throughput might be valuable, it is not the only consideration. Attributes such as safety, availability, reliability and quality of service are also important customer expectations; these are reflected in the need to find the most appropriate capacity balance for each railway operation.


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