That diesel trains emit more pollution than electric trains is no surprise. Perhaps more surprising is that pollution levels inside trains can be higher than those found by city centre roads.

That’s an overall conclusion from research conducted by RSSB early in 2020 with funding from the Department for Transport. Its work prompted a press release on September 16 from Rail Minister Chris Heaton-Harris. “I have asked the industry to immediately conduct further research and explore all engineering modifications and options to rapidly improve air quality on trains and in stations.”

RSSB looked at six types of train running over six routes. This mix included diesel and bi-mode trains. RSSB explained in its 122-page report that it had planned to include another three routes that included two electric trains. However, the pandemic curtailed the work it did between January and March 2020.

The train types ranged from a High Speed Train (a heavy diesel locomotive) to modern bi-modes from Hitachi and Stadler. RSSB chose most routes centred on London because it was easier for its researchers. They ventured between Ipswich and Cambridge to assess Stadler’s Class 755 bi-mode which Greater Anglia uses.

It dropped plans to survey Class 385 (an overhead wire electric multiple unit used by ScotRail), Class 159 (a South Western Railway diesel multiple unit) and Class 444 (an SWR third-rail EMU).

Four pollutants

The researchers looked at four pollutants; black carbon (BC), nitrogen dioxide (NO2) and two sizes of particulates PM10 and PM2.5

Black carbon is the sooty material that diesel engines eject in their exhaust gases. RSSB notes that it can cause lung diseases. Nitrogen dioxide is also a product of burning diesel and can affect peoples’ lungs. The two numbers suffixed to PM relate to the size of particles. The smaller number represents smaller particles which researchers think penetrate further into lungs.

RSSB looked at how gradients and tunnels affected pollution levels inside trains. It looked at whether passengers would receive greater or lesser levels of pollution depending on where they sat.

Pollution comes not only from engine exhausts. Particulates may be generated from mechanical brakes (brake dust) or from carbon being worn from pantographs to give two examples.

An electric train pauses at Chelmsford station.
Electric trains produce some carbon pollution as their pantographs wear but it’s vastly less than diesel trains. PHILIP HAIGH.

Looking for links between different types of pollution gives a clue to their source. If levels of black carbon and nitrogen dioxide correlate, RSSB suggests that the pollution is linked to diesel engines. If particulate measures correlate with each other but not with black carbon or nitrogen dioxide then it’s more likely to come from abrasion such as mechanical brakes or passengers moving around than from burning diesel.

Not just diesel engines

Thus the higher pollution levels on board Class 800s and Class 755s come from diesel engines. Pollution on Class 230s is more likely from brakes or by passengers moving at the route’s frequent stations, according to RSSB. It added that this type’s hard floor might have contributed. This makes it easier for passenger movement to stir particles back into the air. Class 230s also use diesel particulate filters on their engine exhausts. RSSB notes that this is likely to reduce the overall contribution to pollution from their exhausts. 

RSSB reported that Class 221s showed a similar but weaker correlation, perhaps explained by their rheostatic brakes. The ‘221’ recorded sharply rising levels of nitrogen dioxide when it reached Birmingham New Street station. This seems likely to be the result of that station’s enclosed platforms. When monitoring a Class 168 running into nearby Snow Hill station, RSSB noted higher levels of nitrogen dioxide. It said they were “most probably due to emissions from idling trains in the covered station”.

Instruments on board the ‘168’ recorded higher levels of all four pollutants just after leaving and before entering Marylebone station. RSSB’s report says: “The build-up of concentrations in the Marylebone tunnel is quite clear for all pollutants on the journey on 15 January. Concentrations started increasing when the train entered the tunnel just before 1145, with an increase on the NO2 concentrations from 80 to 160μg/m3; 5 to 45μg/m3 (PM10); 5 to 40μg/m3 (PM2.5); and 1 to 12.5μg/m3 (BC).

The effect of the tunnel on the inward journey is also clear, with a build-up of NO2 from 50 to 300μg/m3; from 12 to 80μg/m3 for both PM10 and PM2.5; and from 2 to 35μg/m3 (BC). Since all combustion-related pollutants increased in the tunnel and the PM10 increase equalled that of PM2.5, this might indicate that tunnels have an effect on the accumulation of the diesel-combustion pollutants.”

Pollutions falls

Monitoring a Class 800 running between London Paddington and Bristol Temple Meads allowed RSSB to see the effect of switching between diesel and electric modes. Heading west, the trains switched to diesel at Chippenham where instruments then recorded rising levels of nitrogen dioxide. It commented: “Concentrations of NO2 and BC were very low (<6 and 1μg/m3, respectively) until the train approached Chippenham, where the engine was switched to diesel, and from that moment, concentrations started to elevate. The highest concentrations in this example journey reached in excess of 600μg/m3 (NO2) and 5μg/m3 (BC) measured at Bath Spa and Bristol Temple Meads, respectively.”

Returning east, levels fell after Swindon and the switch from diesel to electric power.

Gradient had less effect than RSSB expected. Running uphill with the Class 221 did produce more pollution. For particulates the difference was not significant says RSSB. For NO2, there was a significant difference with uphill runs producing more NO2. RSSB suggests that the train’s air-conditioning may be filtering out some of the effect of particulates or that running the engines at higher power produces proportionately more NO2 than particulates.

RSSB examined London St Pancras-Nottingham High Speed Trains to see if passengers were exposed to more or less pollution depending on where they decided to sit on the train. HSTs have power cars and thus exhausts at each end of the train. This prompted RSSB to measure pollution in the second coach from the front and second coach from the end. It added a central coach to provide a comparison point. 

The results showed “that the location furthest from the exhaust was highest, while that closest to the exhaust was lowest (and the measurement made in the middle of the train was between the two extremes). This demonstrates that the exhaust does not enter the train until some distance after the exhaust outlet.”

It added: “The train exhaust may be either entering via the air conditioning intake underneath the carriages, or from poorly sealed windows and doors along the length of the train.”

Exhaust makes a difference

Direction did make a difference with Class 755s running between Ipswich and Cambridge. These trains run using diesel when west of Stowmarket. RSSB found higher levels of interior pollution when running from Cambridge because the exhaust was at the front of the train. Nitrogen dioxide levels were 10 times higher, particulates between two and three times higher and black carbon 90 times higher.

Having noted that pollution levels on trains could be higher than on city centre streets, RSSB compiled figures showing how different modes of travel compared. “Air quality on intercity electric and bi-mode trains is clean in comparison to urban travel modes such as cycling and driving; except when in stations alongside other diesel powered trains. Travelling by diesel train, even between cities, exposures passengers and staff to poor air quality at a similar magnitude to that experienced in other forms of transport in the urban environment.”

A version of this article first appeared in RAIL Magazine in October 2021. See more in

By Philip Haigh

Freelance railway writer, former deputy editor at RAIL magazine - news, views and analysis of today's railway.