Railway retrofit10 June 2021

An emissions treatment system adapted from commercial vehicles provides a possible way to improve air quality in the rail network. By James Thorpe, senior sales manager, Eminox

There’s currently an intense focus on reducing emissions and improving air quality. Around 40,000 premature deaths in the UK are linked to poor air quality every year – a shocking total. Across every industry, companies and regulators are therefore looking at how they can reduce emissions to cut pollution. The UK rail sector is no exception. 64% of all energy used to power trains comes from diesel, meaning the rail network is a major contributor to pollution, particularly in and around stations. Demonstrating this, research led by King’s College London found that concentrations of air pollutants, including particulate matter (PM), were 30-40% higher at Edinburgh Waverley station compared to the surrounding area. While new electric and hybrid trains have been introduced, the problem is not going away: emissions from freight trains increased by 8.6% year-on-year in 2019/20, according to regulator the Office of Rail and Road. With just 38% of UK track electrified, it is simply not feasible to phase out diesel in the short to medium term.

The positive news for rail is that cleaner solutions which reduce pollution cost-effectively are available. Essentially the sector can learn from approaches used in other industries with heavy-duty diesel vehicles, such as buses, trucks and construction plant.

Here, collaboration between government, local authorities and industry has provided targeted funding enabling on-road operators to retrofit Euro VI/Stage 5 equivalent emissions reduction technology to existing fleets. This is a cost-effective alternative to scrapping and replacing vehicles. For example, central government recently awarded Transport for Greater Manchester £14.7m to improve air quality by committing to a retrofitting programme of 900 buses. Essentially, for every £1m invested, over 60 buses can be upgraded to the latest emissions standards. With the average cost of a new, replacement bus over £200,000 (and more for a hybrid or electric model), replacing the entire fleet would require an investment of over £180m.

At the same time, local authorities are increasingly introducing Clean Air Zones (CAZ), such as the London Ultra Low Emission Zone (ULEZ), which will be extended to the North and South Circular roads in October 2021, with high daily charges for vehicles that don’t meet emissions standards.

This carrot-and-stick approach results in lower emissions of PM and nitrogen oxides (NOx), significantly improving air quality while preserving investments in existing vehicles. Health benefits are enormous. Analysis commissioned by the Greater London Authority found that its ULEZ will save the NHS £4.17 billion by 2050 as air pollution-related diseases are reduced by 29.5%.

As with buses and trucks, existing diesel rail engines can be retrofitted to reduce emissions in the short term, particularly as mid-life diesel trains are well-engineered vehicles that can be adapted with no impact on performance or reliability.


To explore this potential, emissions control system supplier Eminox worked with rolling stock assets company Porterbrook and South Western Railway on a pioneering proof-of-concept project. Funded by UK government agency Innovate UK, this real-world pilot saw a 1990s-built South Western Railway Class 159 diesel train in full passenger service between Waterloo and Exeter retrofitted with Eminox’s EMx S5 emissions reduction system. (Eminox’s rail experience includes supplying more than 850 emissions control systems for the Hitachi AT300 product family).

When it comes to reducing emissions, diesel trains do have certain requirements, particularly around high exhaust temperatures and changing operating conditions when in use. Rail vehicles’ engines operate at more than 600°C in normal operating conditions; this compares with heavy-duty on-road applications operating temperatures of around 300°C. Because of this, catalyst formulations have to be specifically adapted to suit operation at high temperatures to ensure sufficient NOx reduction.

Eminox’s solution was adapted to meet these needs through the development of suitable on-board urea storage, dosing and air systems, rail-worthy electronics units and advanced catalyst performance characteristics. 3D scanning was used to map out the space to fit the solution within the existing vehicle underframe.

The train’s engine was a 14-litre 400hp Cummins NTA855R3 diesel, and the system was fitted as a direct replacement of the OE silencer, including a new front pipe from the turbocharger.

When in use, the retrofit system works to reduce emissions in multiple ways. An oxidation catalyst cleans the exhaust gas of CO and hydrocarbons, while a wall-flow filter removes particulate matter, including carcinogenic ultra-fine particles. NOx emissions are then reduced through urea-based selective catalytic reduction technology. That consists of injection and mixing of diesel exhaust fluid (AdBlue), followed by a pass through a catalyst. Finally, a clean-up catalyst removes any excess ammonia. The system is actively managed by an electronic control unit, calibrated to maximise emissions reductions over the course of operations. Different catalytic components were used than in the Hitachi case.


The six-month trial clearly showed the potential of retrofit technology in diesel rail, successfully reducing real-world tail pipe pollution. Engine emissions were measured on a test engine on an engine dyno at LH Wabtec. Emissions measurements were taken from all nine operating modes of the engine, from notch zero (idle) through to notch seven: peak torque and peak power.

NOx emissions dropped by over 80% and there was a 90% reduction in hydrocarbons, carbon monoxide and PM. This brings emissions performance in line with international Stage V standards. Most importantly it proved that the technology works in the real world, over continuous operation, with no impact on cost or performance.

In particular, during back-to-back tests on the engine test bed facility at LH Wabtec, 75% of the operating conditions saw a reduction in fuel consumption, with only the peak torque condition seeing any significant increase. Results were independently verified by Ricardo, and were in line with other back-to-back trials.

The rate of consumption of diesel exhaust fluid, AdBlue, was approximately 3l/hour when at operating conditions, and 2l/hour overall (which includes idle time). Over the length of the trial, the system used more than 9,500l of AdBlue.

Recognising the trial’s success and the potential for future deployments, the project won in the Sustainable Business category at the National Rail Awards 2020.

Now is the time to build on the proven benefits of the trial and use it as a blueprint for a wider retrofit programme across UK rail. It provides a cost-effective approach that can be rolled out across the large number of similar diesel trains that are currently in service, reducing pollution, improving air quality, and creating healthier environments.

Given that UK government has committed funding of over £110m to retrofitting buses, there is a proven model in place that can be replicated in rail. This will help the industry cost-effectively create a fleet of clean diesel trains that can remain in service well into the future.

James Thorpe

Related Companies
Eminox Ltd

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