The importance of slipstream safety
Source: RTM Feb/Mar 17
Simon Lomax, inspector at the Rail Accident Investigation Branch (RAIB), discusses the importance of communicating sufficient slipstream safety information at stations following last year’s accident at Twyford.
Readers will probably have stood on a station platform as non-stopping trains pass and experienced train slipstreams for themselves. The hazard such slipstreams pose to passengers and their belongings have been known for many years. In the UK, concern about train slipstreams began in the 1970s instigated by a general increase in train speeds.
A series of slipstream-related incidents occurred on the then ‘Southern Region’ involving trains conveying road vehicles from a factory near Southampton. Subsequent research led to a greater understanding that even relatively slow-moving freight trains could produce slipstream effects that were more severe than those from much faster-moving passenger trains. Why? Because freight trains are aerodynamically rough, unlike passenger trains which tend to be relatively smooth.
The slipstream hazard was brought into sharp focus on the morning of 7 April 2016. A woman was making a trip with her wheelchair-bound daughter from Twyford to London. The woman stopped to buy a coffee from a kiosk on the station platform. She positioned her daughter, in her wheelchair, well behind the platform yellow line and applied the wheelchair brakes. Despite these precautions, the slipstream generated by a freight train passing at only 45mph was sufficient to cause the wheelchair to start moving and into multiple glancing contacts with the passing wagons. By good fortune, the wheelchair was pushed away from the train by the last contact and her daughter sustained only minor injuries.
The RAIB investigated this accident because it could see the potential for safety learning. The investigation included full-scale measurements of train slipstreams at Twyford. This was carried out using sonic anemometers on the platform; these measured the airflow as trains passed. Airflows within train slipstreams are highly turbulent and complex, and hence it was necessary to use statistical methods when evaluating the forces which would have been exerted on the wheelchair. The testing process demonstrated that the forces generated by the train’s slipstream, combined with the ambient wind on the day of the accident, would have been sufficient to overcome the wheelchair brakes.
A key factor in this accident was that the mother did not appreciate that a freight train, moving at a relatively modest speed of 45mph, could have been such a threat to her daughter. This is, perhaps, quite understandable. An HST passing at 100mph or more is clearly something to be wary of, whereas a slower freight train may appear to be more benign.
The RAIB investigated what measures the railway industry should, and could, take to ensure that station users are made aware of the potential hazards. Many stations, including Twyford, have yellow lines on the platform and instructions to stand behind them. But, in this case, the wheelchair was parked well away from the yellow line, so clearly this measure is not always adequate. Another approach is to use announcements to warn of approaching non-stopping trains. However, Twyford had many such trains, and so there were many of these announcements – including announcing trains passing platforms to which the public did not normally have access. This meant that the warning messages lost their impact. The RAIB also found that important findings from previous research into freight train slipstreams had not been reflected in current standards.
The accident at Twyford illustrates the importance of informing the travelling public about hazards on the railway. The public should rightly be expected to behave in a safe manner, but it is incumbent on the industry to make them aware of hazards and provide them with sufficient information to be able to take adequate precautions. That information should also be provided in an appropriate and accessible manner, whilst avoiding information overload which can have the opposite effect to that which was intended.
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