Emission factors are a key tool for estimating emissions of pollutants but those currently used to estimate GB rail emissions are out of date and provide a poor representation of how emissions vary according to engine operating condition. In 2018, RSSB established an Air Quality Steering Group comprising members from across the rail industry. The group launched the Rail Air Quality Strategy in 2020 which is underpinned by the Clean Air Research Programme (CLEAR). 

The CLEAR programme incorporates robust research to measure air quality on the rail network and gain a better understanding of rail’s contribution to local pollution levels. This research also supports the industry in establishing a ‘baseline’ from which improvement measures can be implemented and evaluated against. 

One of the programme’s projects was RSSB’s ‘CLEAR: Fleet wide assessment of rail emission factors, T1187’. Its main objective was to develop a set of new emission factors for the GB rail fleet so that national and local impacts of rail emissions could be more accurately assessed. 


There is a need to improve estimates of emissions from the rail sector nationally. Improved emission factors are also required to better understand air quality impacts in local areas, such as stations and urban rail freight yards. This provides the rail industry with the data necessary to respond to future regulatory requirements and provide policy makers with robust data on which they can base strategy decisions.

In previous versions of the UK National Atmospheric Emissions Inventory (NAEI), NOX emissions from both passenger and freight diesel trains were estimated to account for 4% of the UK’s total NOX emissions. However, these estimates were based on work that had been conducted around the year 2000 and had not been updated. 

However, since that time, certain locomotives have been re-engined. Importantly, various new passenger trains have also been introduced as well as new locomotives which did not have specific emission factors developed for them at the time of their introduction. Existing factors also gave an over simplistic single number representation of emissions per distance travelled which did not reflect the impact of engine operating cycle (accelerations, decelerations, idling) on harmful pollutant emissions.

Key findings

Emission factors by notch have been developed for various diesel train classes. Using the new emission factors developed, the total national rail emissions estimates in the NAEI has been adjusted. The estimated NO2 and PM10 emissions from rail have reduced by 34% and 64%, respectively, from the 2017 to 2018 releases of the NAEI, as a result of this work.

Apart from the impact on the estimate for national emissions, further analysis using the new emission factors on a local level can be carried out. When combined with detailed on-train monitoring recorder (OTMR) data for specific routes, emission factors by notch can be used to demonstrate the sensitivity of emissions of NOX, PM and CO2 to various operational factors (such as the number of stops and variations in loading) for both passenger and freight trains.