GWR Hitachi train at Truro

UK: The Department for Transport’s Science Advisory Council has made a number of recommendations in response to a Rail Safety & Standards Board analysis of air quality onboard trains.

The council notes that the methodology makes it difficult to compared on-train air quality with outdoor observations, and there is a lack of data for equivalent car or bus journeys. This makes it difficult to evaluate how exposure to air pollution on trains might compare with counterfactual journeys.

RSSB found that particulate matter concentrations were frequently higher inside trains than would be experienced outdoors, but the council says these were ‘broadly within the range of interior concentrations reported previously in literature’.

The council says ’it may be worthwhile to follow up with additional analysis to identify links between PM and known causal factors such as type and effectiveness of engine aftertreatment system, type of braking system, interior air exchange rate and passenger occupancy level’.

Unexpectedly high concentrations of NO2 found on Class 800 electro-diesel trains in diesel mode ‘require further investigation’, as the selective catalytic reduction ‘may not be functioning as anticipated and/or the air intakes to some passenger compartments may not be optimally configured, unintentionally drawing in engine exhaust’.

Further analysis of airborne particulates inside trains would support targeted interventions to address the most harmful components.

The council says the performance of future propulsion technologies and ventilation systems should be evaluated for their effects on train interior air quality. It notes that a transition away from diesel would eliminate the risk of elevated NO2 inside trains, and that all-electric trains also emit fewer particulates.

It says any long-term strategy which retains combustion propulsion using alternative low carbon fuels such as hydrogen, ammonia or biodiesel could potentially lead to continued emissions of NOx. This would necessitate the use of either well-optimised combustion conditions — which ‘may work for H2, but likely not for biodiesel’ — or the continued use of exhaust gas aftertreatment.