At a glance
• Leaf fall is resulting in closures on the Piccadilly line between Rayners Lane and Uxbridge to prevent wheel damage
• Leaves create a slippery film on tracks, causing trains to skid, flattening wheels and disrupting signalling
• TfL combats this with rail adhesion (“RAT”) trains that spread a sand compound to help Tube trains stick to the track
If you’re wondering why your Piccadilly line train has been delayed or cancelled recently, the reason may come as a surprise: leaves on the line.
What many people don’t know is that the London Underground has more tracks above ground than in tunnels, meaning that about 55 per cent of the Tube is open to the elements.
Transport for London has occasionally suspended services at the western end of the Piccadilly line on a temporary basis since October – with more shutdowns likely to continue on an ad hoc basis until January.
For several days at a time, there may be no Piccadilly line trains between Rayners Lane and Uxbridge – and only a limited shuttle service between Rayners Lane and Acton Town, where passengers may have to change for onward services to central London.
Passengers for stations between Rayners Lane and Uxbridge can use the Metropolitan line – but may face a longer wait for a train than normal.
TfL has taken the proactive approach this autumn and winter in a bid to prevent damage to the Piccadilly line that saw many trains having to be taken out of service last year.
Of all the Tube lines, the Piccadilly line has the biggest problems, mainly because its trains are 50 years old and don’t have wheel slip protection, but also because of the heavily tree-lined routes it follows in west London.
When the trains lose adhesion, they ‘skate’ or skid on the track – and this causes flat patches to develop on their steel wheels. This causes the wheels to become mis-shapen and requires the trains to be taken out of service.
TfL says the problem with leaves on the line is akin to what would happen if a car was braking on an icy surface.
Depending on the amount of damage, the train wheels can be re-shaped by running the train through a giant lathe, or by splitting the trains in two, lifting them in a depot, then removing the bogeys from the trains and replacing an entire set of wheels.
But both types of repair are time-consuming – which is why TfL has been more pro-active this year and has closed sections of the Piccadilly line at times of greater leaf fall, knowing the damage awaiting its trains if it keeps them in service.
Four lines suffer most from leaf-fall problems: the Piccadilly, the Metropolitan, the Central (at its western and eastern ends) and the Northern, on both its Edgware and High Barnet branches. The worst year was 2016.
But TfL is doing more than just part-closing lines. It also operates five “rail adhesion trains” – nicknamed “RAT” trains - to lay a sand compound on the tracks to break down leaf mulch and help the train wheels stick to the tracks.
These RAT trains are repurposed vintage Tube trains that are likely to cause passengers do a double-take if they seem them at work on the Underground during the day.
The RAT trains operate during normal hours on the Metropolitan line – in between normal services - and mostly in “engineering hours”, namely overnight, on the Piccadilly, Northern and Central lines.
Network Rail has an alternative approach and runs a locomotive on its tracks, which share many of the same corridors as the Met line, that is equipped with high-pressure water jets to blast the leaf mulch from the rails. TfL has a similar device that it uses at night to “power wash” the rails.
TfL records the types of trees that are planted beside its tracks. Of these, 85 per cent of trees that shed their leaves early have done so, but only 50-60 per cent of “late” trees, such as tall oak trees.
The RAT trains spread a compound known as sandite – a mixture of sand and metal - onto the rails. This is ground into the leaf mulch by the RAT train’s wheels, and by the wheels of following trains, and breaks up the mulch.
The RAT trains have been fitted with giant “hoppers” that are loaded with sandite, which is released onto the tracks via holes drilled in the train floor.
The Met’s RAT train is a D-stock that used to run on the District line. The Piccadilly RATs are two three-carriage units. The Central line RATs are two 1962 trains.
Richard Jones, director of asset performance delivery for London Underground, said fallen leaves formed a “film” on top of the tracks that made them “very, very slippery” and caused trains to skate.
It also causes problems with signalling systems, by preventing an electrical connection between the train wheels and the track.
Mr Jones told The Standard: “The service interruptions that we have put in place on the Piccadilly line are to protect the [train] wheels.
“Where we have got the option to suspend the service on the Piccadilly line, but the Metropolitan line covers it, for example between Rayners Lane and Uxbridge, we can do that.
“It means that we can protect the wheels on the trains that are most vulnerable, which are our 50-year-old-plus Piccadilly line services.”
He added: “Last year we experienced an awful lot of wheel damage, which was very disruptive. We took an awful lot of trains out of service.
“We put a strengthened plan in this year, to make sure that we try not to get into that situation again.”
TfL also dispatches leaf collectors, gangs of workers to pick up leaves overnight. “We have collected nearly 10 tonnes of leaves this year,” Mr Jones said. “That is to stop the dead leaves blowing around and ending up on the track.
The Met line, while being “very leafy” as it approaches Amersham, Chesham and Watford, does however benefit from more modern S-stock trains, which have wheel-slip protection.
“It’s still a challenge,” Mr Jones said. “That is why we need to run a RAT train at the northern end of the line. Once you get past Rickmansworth it gets very leafy.”
Temporarily closing the Rayners Lane to Uxbridge section of the Piccadilly line has benefit of increasing the number of trains available on the rest of the line, which carries 10 per cent of all Tube passengers.
Weather forecasts are checked twice a day to assess the most useful times to suspend services.
Each day is colour-coded green, yellow, red or black, the latter being the “absolute worst” conditions. “The last two weeks have been particularly challenging,” Mr Jones said.
The delayed arrival of the new fleet of Piccadilly line trains – the first is due to enter service in the second half of 2026 – should ease the problem, as they will be fitted with wheel slip protection, but not eradicate it.
Snow and ice cause a different problem. Then the issue is ice forming on the conductor rail – the “third rail” that delivers power to the trains. The ice pushes up the “collector shoe” – the device under the train that draws power from the conductor rail, and prevents power reaching the train.
“Rail adhesion is an issue that will never go away,” Mr Jones said. “But a more modern train that has got systems on it that protect the wheels, and stop wheel slip and slide, will make it a lot better.”
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