Rail is often hailed as one of the low carbon ways to travel, and rightly so. But while the sector leads on climate performance, improving air quality (AQ) within rail environments remains a critical challenge.
Air Quality vs Whole-Life Carbon: What’s the Difference?
While they’re often discussed together, AQ and WLC target different, but equally important outcomes:
- Air Quality refers to the immediate impact of pollutants like nitrogen dioxide (NO₂) and particulate matter (PM) on human health. These pollutants are strongly linked with respiratory and cardiovascular conditions.
- Whole-Life Carbon considers the total greenhouse gas emissions of an asset over its lifecycle, including manufacturing, maintenance, operation and disposal, which contribute to climate change. Both require urgent action, but different tools.
As trains pass through enclosed stations or operate in diesel mode, they can expose passengers and staff to harmful pollutants, even as their whole-life carbon (WLC) performance may be improving. We have explored methods which have described how emissions in the rail sector can be reduced.
Improving Air Quality in the Rail Environment
Work published by the Rail Standards Safety Board (RSSB) provides a toolkit of proven approaches to reducing emissions and exposure in the rail environment. These include:
- Tackling Emissions at Source
Where feasible, electrifying trains or deploying battery-electric units cuts emissions almost entirely. But even when this isn’t possible, significant improvements can be made through:
- Retrofitting diesel fleets with modern exhaust after-treatment systems (EATS) such as diesel particulate filters or exhaust gas recirculation, cutting NOx by 75% and PM by over 80%.
- Replacing older engines with newer, cleaner models stricter emissions standards (applicable at the point of entering the market) for NOx and PM set in retained EU law.
- Installing start/stop technology to avoid unnecessary idling, especially in enclosed stations or depots.
In San Francisco, electrifying a commuter rail line reduced black carbon exposure on board by up to 89% – a change with clear parallels for UK rail, particularly in enclosed stations or in the area surrounding remaining routes where diesel railways are used.
- Smarter Train Design
Legislation and guidance, such as the Health and Safety at Work Act, require for poor air quality to be treated as a risk to be reduced as far as reasonably practicable. Technical studies by RSSB show that even the positioning of exhaust outlets and heating, ventilation and cooling (HVAC) intakes can significantly impact on-board air quality. Adjustments to recirculate air in tunnels, limit ingress of polluted air, and optimise ventilation in high-traffic areas can all reduce exposure.
- Control of Exposure Within Trains
High-grade filters (e.g. G4 or better) in HVAC systems can remove harmful particles within trains. However, legacy HVAC systems may require upgrades to handle finer filtration without increased wear or reduced airflow.
- Operational Measures
At Birmingham New Street, installing smart ventilation systems using 100 new NO₂ sensors installed in the enclosed platforms helped reduce NO₂ levels by up to 42% and PM by 81%, by controlling when fans were operated to assist with dispersion. These changes not only protect health, they enhance the passenger experience too.
Embedding Circular Economy to Reduce Carbon and Air Pollution
Clean air isn’t the only goal. There are strong links between AQ and circularity. Reusing rail materials can reduce emissions at every stage – from extraction to processing and transport. For example:
- Network Rail’s reuse of ballast is hoped to save up to 500,000 tonnes of virgin material annually.
- Dutch Railways (NS) has achieved a 96% reuse rate of rolling stock components.
- SNCF and Saarstahl Rail now produce low-carbon rails with 70% less CO₂e, made from old rails using electric arc furnaces or reusing them.
Our Consultancy’s Value
As specialists in both air quality and carbon, we help clients apply circular economy principles early in project design, reducing embedded emissions and improving environmental outcomes. We’re also experienced in undertaking air quality monitoring surveys for ambient and indoor settings. Our portfolio includes extensive work in the rail sector, including ambient monitoring for High Speed One construction works throughout London.