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Signal engineers and train operations staff often misunderstand each other when talking about headway.

When someone in the operations team refers to headway, they actually mean the interval between trains expressed in minutes. They assume that the interval between trains is enough to deliver a reliable on-time service.

Signal engineers however calculate headway as the absolute minimum time between following trains that will allow drivers to retain line speed without having to apply brakes due to passing yellow signals.

The signal design will generally try to space signals so that there is a fairly uniform headway across a section of line. The worst headway on the line sets the "ruling headway" for the line. This is sometimes called the theoretical signalling headway. Trains travelling closer than the ruling headway will meet at least one yellow signal and be forced to apply brakes, and will therefore lose time. This in turn will delay the following train and so on, causing cascading and compounding delays.

Several factors contribute to achieving reliable train frequencies, such as the permitted line speed, driver behaviour, train acceleration & braking rates, train length, signalling principles (such as overlap length), planned station dwell time, and most importantly, passenger behaviour.

This paper provides a brief background on classical headway theory; some insight on how track speed and station dwell time impact on achievable capacity; a case study to demonstrate that terminal stations may pose a greater constraint on capacity than the signalling; and a suggested method to allow quick assessment of achievable capacity on a new line.