In rail transport, the distance required for a train to come to a stop is longer than the distance the train driver can see. This is necessary to inform the train driver about the limit of movement, thereby giving enough time and distance to reduce the speed of his train before it reaches the protected point, which can be a point, a station, a level crossing, or another train, etc. Moreover, as the consequence of an unauthorised train movement can be a severe collision or derailment, it is vital to keep trains within their movement limits.
Since the invention of rail transport, the most common communication form between the train driver and traffic management is using railway signals. In the past, form signals or semaphores, and later coloured-light signals were used; nowadays electronic on-board devices are used to perform this function. The messages conveyed by signals are simple: the train driver is allowed to pass the signal or not. It is a simple communication process in which the sender is the traffic controller, the message is the permission or limit of the train movement, the used code is the aspect of the signal, and the receiver is the train driver. This process sometimes fails, and the train driver does not stop the train before the signal, which shows a danger (stop, or red) aspect called signal passed at danger (SPAD).
There are many types of SPAD, depending on the nature of the incident. Among others, we distinguish between en-route SPAD, when the train has been in motion for a long time, and departure SPAD, when SPAD occurs on departure from the station. There are many good and useful tools, devices and systems to help you avoid SPAD, but it still happens from time to time.
Many different factors can lead to SPAD, e.g. brake efficiency, signalling failure, and other vehicle issues. Still, the most common one is the human factor, as the train staff, including the train driver and the crew, has a massive influence on the train movement. It is easy to say that the cause of SPAD is human error, but this does not reveal the real causes of the events. Fatigue, vigilance, communication, ergonomics, workload, and teamwork can influence humans' performance - it is always worth looking at what led to the decisions and actions of the staff involved.
A train driver surrounded by many factors influencing his performance.
The Australian Transport Safety Bureau recently published a final report about a SPAD that occurred in 2019. A passenger train leaving a station passed a signal indicating a danger aspect and almost collided with another passenger train. The report identifies the infrastructural, organisational, and human factors that led to this incident and makes safety recommendations. I find it very interesting how many factors need to be present simultaneously for a decision to be considered erroneous - in hindsight.
If you are interested in rail incident investigation, in the second week of our Applied Rail Accident Investigation course, we will cover in detail the background and investigation of SPAD among several rail operational incidents.