Platform Aerodynamic Risk Assessment
What is aerodynamic risk?
As a train passes through a platform and pushes the air in its path over and around the train, it causes a slipstream alongside. Individuals standing on the platform can feel strong gusts of wind, objects may be swept along the platform or into the path of the train and it may cause vulnerable individuals to lose their balance.
The video below demonstrates this.
Despite the pushchair in this video being as far back from the platform edge as possible, the change in air flow was powerful enough to draw it into the path of the train.
To quantify this risk, the PARA assesses a range of factors, including:
- Speed of the passing train. The faster the train is moving, the greater the disruption to the airflow around the passing train as it pushes air out of the way of and along the direction it is travelling in.
- Distance between passengers/objects and the side of the train. The closer passengers and objects are to the side of the passing train, the greater the changes in air speed will feel.
- The rolling stock type. Some rolling stock are designed to be, or are naturally, more aerodynamic than others. The shape and smoothness of the train exterior impacts the gustiness generated, and the lateral extent of the slipstream caused by the movement of the train. Freight trains, like the one in the video, pose a greater aerodynamic risk when passing than passenger trains for this reason. The highest risk comes from freight carrying exposed loads such as road vehicles, or from container trains with large gaps between the containers due to the inconsistency of the train profile.
- Wind speed and direction. Strong winds can enhance the aerodynamic effects of passing trains. Cross-wind in particular can increase the aerodynamic risk, as slipstream speeds have been found to increase on the leeward side of the train (opposite side of the train to the way in which the wind is blowing).
The assessment also considers other circumstances that may influence how close passengers stand to the platform edge, or when the likelihood of lightweight or wheeled objects being on the platform will be higher. The station may encounter larger crowds during special events for example, placing people closer to the platform edge or see an increase in luggage and pushchairs during school holidays.
By assessing these factors an aerodynamic risk score for each platform is created. This score is then used to determine suitable mitigations.
When is an assessment required?
PARAs are completed for all platforms that have trains passing through them that are not scheduled to stop. This can include both sides of narrow island platforms, even if non-stopping traffic only passes one side of the island. This is because, if the width of the island is very narrow, passengers waiting on the adjacent platform may still be at risk from aerodynamic effects.
PARAs are not required for bay platforms or platforms where all traffic stops at the platform. In these instances, the speed of the train is not fast enough to create sufficient aerodynamic risk that requires assessment.
Completing the risk assessment
The PARA methodology, forms and guidance can be found in Appendix B of Rail Industry Standard Interface between Station Platforms, Track, Trains and Buffer Stops (RIS-7016-INS). RSSB has also developed a platform aerodynamic risk assessment tool (PARAT) which is available for members.
The following information summarises the main steps to complete a PARA.
- Aerodynamic risk assessment data collection. Specific information is required to produce a 'Total Platform Unmitigated Risk Score' for each platform. An aerodynamic risk assessment data collection form is provided in Appendix B section B.1.12 of RIS-7016-INS. Example input information includes:
- Geographical location
- Platform layout
- Number of stopping trains per day
- Type, number and speed of passing trains
- Calculate Total Platform Unmitigated Risk Score. An individual risk score is calculated for each passing traffic type using the data collected. The forms provided in Appendix B section B.1.13 and B1.14 of RIS-7016-INS should be used.
These risk scores are then added up to calculate the Total Platform Unmitigated Risk Score. This score gives an indication of the unmitigated level of risk due to the aerodynamic effects of passing trains at the platform.
Table 3 in Appendix B is used to categorise the score and determine if the platform is low, medium or high risk.
- Determine mitigations. This risk categorisation is then used to help decide what mitigation measures need to be considered for implementation. Mitigations measures are set out in Table 4 in Appendix B, with guidance indicating which are suitable for each risk categorisation.
- When determining the suitability of mitigation measures the following need to be considered:
- Whether it is also relevant to controlling other, non-aerodynamic, platform risks
- Whether it is most effective when used in conjunction with other mitigation measures
- How difficult the measure is to implement
- How costly it is to implement or maintain, and any ongoing costs
- How effective the measure is in reducing aerodynamic risk
- Record assessment results. Using the forms set out in Appendix B will ensure that a historical record is kept of the assessment, the decisions made, the controls already in place and those that will be put in place. This record will inform future PARAs and may be of use in case there are future slipstream incidents.
- Monitor implemented controls. Monitoring helps to determine the extent to which mitigations are working, for example:
- Are the controls helping to reduce the likelihood and severity of an aerodynamic hazardous event, so far as is reasonably practicable?
- Do any controls create risk elsewhere, for example, is the yellow line placement creating crowding issues
- Review risk assessment. Risks may change over time, so the risk assessment should be reviewed to determine if current controls are still valid or if enhancements are needed.
Many things can initiate a review such as:
- Proposed changes to the use of a platform or passing train traffic patterns
- Changes to traffic levels, which might result in a short-term increase in aerodynamic risk, e.g. infrastructure work or possession activities.
- Changes in passenger numbers and characteristics during peak times, or short-term busy periods (such as special events).
- An aerodynamic incident
The Taking Safe Decisions framework guides also provides a process for monitoring safety and implementing changes.