First published in Issue 150 (April 2017) of Rail Engineer.

As a small child, those dire parental warnings about standing too close to the platform edge instilled an inbuilt respect for passing trains that still resonates today. There was, apparently, a grave danger of being ‘sucked in’ … or indeed ‘off’ as the infamous signs at Penrith railway station once proclaimed.

Yes, albeit briefly, those unfortunately worded notices really did exist! Having gone viral on the Internet, they weren’t long in disappearing, but was their intended message based on fact? Could the draught from passing trains really blow a person off their feet?

The short answer is yes.

Prior to the introduction of high-speed trains in the 1970s, British Rail conducted research into the possible effects of slipstreams on station users and trackside staff. It was continued by BR during the1980s and has been taken forward by other organisations to the present day.

One result has been the familiar yellow warning lines painted along station platforms. Usually positioned 1500mm from the platform edge, these lines are mandatory if trains pass at more than 100 mph, or freight trains at more than 60 mph. At the time of their inception, they were intended to mitigate the ‘effect on the elderly’. In more recent times, the body of research has indicated that it is possible for a freight train slipstream to move a pushchair with its brakes applied.

On 7 April 2016, a mother and her disabled daughter had decided to have a day out in London. It’s a 50-minute train ride from Twyford station in Berkshire to London Paddington. They arrived onto Platform 4 at Twyford with time to spare, so the mother carefully parked her daughter’s wheelchair, well behind the yellow line, parallel to the platform edge facing towards London. Her idea was to shield her daughter from the dust generated by passing trains.

Two minutes later, train 7A09, the 07.12 service from Merehead in Somerset to Acton Yard in West London, passed Platform 4 at approximately 45 mph. It comprised a Class 59 locomotive drawing 16 loaded box wagons (types JNA and JYA) followed by 28 loaded hopper wagons (types IIA and JHA).

About 13 seconds after the locomotive had passed, the girl’s wheelchair began to move, turning towards the right and then approaching the train. Six seconds later the wheelchair’s right footplate contacted the twenty- seventh wagon. This caused the wheelchair to rotate and move parallel to the train. It then struck the twenty-ninth wagon, rubbing along the side of it before being contacted by the leading edge of the thirtieth wagon.

The impact propelled the wheelchair along the platform and away from the train. It came to rest about 15 metres away from its starting point. A very frightening experience for the girl, but fortunately she’d had a lucky escape.

The accident was witnessed by a member of the GWR ticket office staff and a member of the public, both of whom ran to help. The mother had been looking in the opposite direction and was initially unaware of what had taken place. When she turned round and noticed that the wheelchair had moved she also rushed to the aid of her daughter.

The girl had a minor injury to her foot and there was some damage to the wheelchair. The mother and daughter subsequently boarded the 10.56 GWR service to Paddington.

The GWR staff member who noticed the accident reported it immediately to GWR Control, which then reported it to the Rail Accident Investigation Branch (RAIB). The mother and girl were met at Paddington by GWR staff who checked on their welfare and gathered some basic information about the accident. Had the staff member at Twyford not been vigilant, it is doubtful that the subsequent RAIB investigation would have taken place.

RAIB involved

Simon Lomax is an Inspector of Rail Accidents, based at the RAIB offices in Derby. He was a key member of the team who worked on the Twyford accident report.

“Initially we had very little information – the wheelchair had rolled for reasons unknown,” he said. “We looked at the CCTV recording from Twyford and we had the statement taken by the GWR staff at Paddington. It was tempting to leap to a conclusion that the mother had not applied the wheelchair brakes.”

Simon then went on to explain that, even if that were the case, the public needs to be made aware of potential issues caused by train slipstreams. “People will be wary if they see a train approaching at 125 mph, but a freight train moving at 45 mph probably doesn’t give any impression of danger.”

As it was, the mother was keen to cooperate with the RAIB and remained adamant that the wheelchair brakes were, in fact, applied. It became apparent that a scientific approach would be needed in order to establish the processes that led to the accident.

Testing

Simon explained that the RAIB’s first objective was to measure the force needed to overcome the wheelchair brakes. “That was easy enough, but more challenging was the need to assess the forces that can be generated by train slipstreams.”

Professor Chris Baker at the University of Birmingham has undertaken considerable research in the field of vehicle aerodynamics and environmental fluid mechanics. At the start of his career, he had worked in the Aerodynamics Unit of British Rail Research Division in Derby. He offered his expertise to the RAIB enquiry and was invaluable in setting up a test plan.

Ultrasonic anemometers, which can measure turbulent air flow in three dimensions, were set up at Twyford to measure the slip stream effects of passing freight trains. Four such devices were deployed, 17 metres apart and 2.4 metres from the platform edge – the same distance from the edge as the wheelchair before it started moving. The experiments were undertaken at night, not only to minimise disruption to the station users, but also because ambient air speed tends to be reduced at that time. A radar speed gun was used to measure the speed of passing trains. Data was recorded from 28 passing trains, some of which had consists similar to 7A09 on 7 April 2016, including the transition from box wagons to hopper wagons.

Slipstreams generated by trains are governed by chaos theory. There is an interesting difference, though, between the slip streams generated by passenger trains and freight trains. Passenger trains, being relatively smooth sided, tend to generate less turbulent air movements than freight trains.

When a fast moving passenger train passes, there is an initial pressure wave that tries to push objects away. This is followed by a period of reduced and benign slipstream air flow and then finally there’s a turbulent inrush of air behind the train that creates a suction effect towards the track.

Freight trains tend to have gaps between vehicles or containers and are generally ‘lumpier’ than passenger trains. As a consequence, numerous vortices or swirls are generated, which can interact with each other in complex and unpredictable ways – sometimes adding in strength and sometimes subtracting.

Probability

As Simon explained: “The occurrence at Twyford was a rare event, which makes it more difficult to understand. However, the statistics of slipstream turbulent flow indicate that the longer a freight train is, and the faster it moves, the greater probability there is that vortices can combine in a manner to create a strong wind gust; sufficiently strong to move a wheelchair against its brakes.”

At Twyford this appears to have happened at the transition between the box wagons and hopper wagons of train 7A09, just as they were passing the girl in the wheelchair. A useful visual indicator on the CCTV recording was provided by the long blond hair of a lady standing nearby.

Of course, moving air must act against an object in order to create a force. The force exerted depends on the object’s surface area perpendicular to the slipstream, its coefficient of drag, the density of the air and the square of the wind velocity. The RAIB investigating team was able to draw on research undertaken by NASA to determine the coefficient of drag for the wheelchair.

The wheelchair used by the girl was of a type that can only be moved and braked by a carer. It was fitted with two drum brakes, each operated by its own hand lever.

As part of its product acceptance process, this wheelchair design was tested in accordance with the applicable standards. The test report, dated 2004, showed that the wheelchair brake force was 239 N. RAIB testing of the Twyford wheelchair, which was six years old, found that the brake effectiveness was significantly less; probably just 87 N at the time of the accident. Calculations undertaken by the University of Birmingham showed that this would have been insufficient to resist the peak aerodynamic forces acting upon the wheelchair.

The wheelchair involved in the accident. (Inset) The wheelchair brake mechanism.

To makes matters worse, as the wheelchair approached the platform edge, the slipstream forces acting on it would have increased. At 1.2 metres from the platform edge – the position of the yellow line on Platform 4 – it was calculated that the slipstream force would have increased by around 2.7 times.

Accident factors

Ultimately, it was deduced by the RAIB investigators that three factors came together to cause the Twyford accident. The first was the effectiveness or otherwise of the wheelchair brakes. Investigation found that the braking effect on one side of the wheelchair was weaker than the other, which explains why it veered towards the train.

The second factor was the turbulent slipstream generated at the area of the change from box wagons to the taller hopper wagons.

The third was the ambient wind speed and direction. Data from three local weather stations confirmed that it was blowing at 2-3 metres per second in broadly the same direction as the train movement. The University of Birmingham was able to recreate these factors and concluded that it was ‘plausible’ that the combined wind effects overcame the wheelchair brakes.

Surprising

Set up in 2005, the RAIB independently investigates accidents to improve railway safety, and inform the industry and the public. Whether the wheelchair brakes had been on or off, the RAIB would still have needed to produce recommendations. That the wheelchair brakes could be overcome was however a key realisation.

A previous RAIB accident report, ‘Accidents at Southend Central and Whyteleafe Stations’, had looked at incidents of wheelchairs and pushchairs rolling over platform edges. In those accidents, the cause was gravity and platforms that sloped towards the railway, but the resulting recommendations were still about how the railway assesses the risks to vulnerable users of stations.

The railway does of course have a duty of care to all station users, yet in carrying out risk assessments on stations it seems that the potential effects of train slipstreams have not been properly appreciated. Simon expanded on this: “The railway has lost sight of the risks from train aerodynamics. That a freight train at 45 mph can generate a slipstream that is worse than that of a passenger train travelling at twice the speed has not been understood. There is therefore a need to raise the consciousness of station operators and duty holders to train slipstream effects.”

Unaware

The mother involved at Twyford was unaware that an accident like this could happen, and was very clear that she didn’t want anything like this to happen to anyone else. Fortunately, the accident did not result in serious injury, but it could have been much worse. Indeed, the girl was very lucky to survive her encounter with the moving freight train.

No aerodynamic risk assessment had been carried out at Twyford Platform 4, because it wasn’t required under Railway Group Standard GI/RT7016 ‘Interface between Station Platforms, Track and Trains’ Issue 5 March 2014. That standard only requires such an assessment if passenger trains pass at speeds greater than 100 mph, or freight trains at more than 60 mph. The maximum speeds at Platform 4 are 90 mph and 60 mph respectively, so there was no formal need to mitigate the risks from passing trains.

The wheelchair was positioned behind the yellow line by 1.2 metres, so it’s now clear that this measure alone does not ensure safety. The mother was unaware of the hazard presented by the slipstream of passing trains, so she didn’t take any additional precautions. In any case, the RAIB investigation found that there was a lack of clarity about what specific actions should be taken by station operators and wheelchair users to minimise the risk.

Changes

The RAIB is not an enforcement organisation. It was set up to provide safety learning and it falls upon others to act upon its recommendations. Those recommendations are crafted very carefully. After all, a recommendation that can’t be implemented has little value. For instance, a blanket low speed limit on freight trains passing through station platforms would be unacceptable. Rather, the recommendations need to be appropriate and proportionate and, in this, the RAIB can claim that 97 per cent of its recommendations lead to substantive actions being taken.

The published Twyford accident report recommends that the railway industry should make members of the public aware as quickly as possible of the potential hazard from train slipstreams and of the need to keep hold of pushchairs and wheelchairs. It calls upon the Rail Delivery Group to work in consultation with passenger groups to seek ways of providing advance information (published advice leaflets) and real time information (announcements and customer information displays). It has also called upon the Railway Safety Standards Board (RSSB) to work with the rail industry to identify consistent measures that can be taken to ensure the safety of wheelchairs and pushchairs from train slipstreams.

Since the incident at Twyford, GWR has started a project to carry out aerodynamic risk assessments for all its station platforms and it will review how it warns station users of the approach of passing trains. The RSSB has started a process to amend Railway Group Standard GI/ RT70016 to make it mandatory for aerodynamic risk assessments to be undertaken when freight trains pass platforms at 45 mph or greater.

Written by Stuart Marsh.