On 20 November 2016 Londoners awoke to find their city battered by torrential rain and winds of up to 100mph. Storm Angus, which had spent the previous day causing chaos to shipping in the Atlantic and across the western coast had finally reached the city.

Angus fled the Capital almost as fast as it arrived, leaving a trail of minor flooding and train delays in its wake. This was seen as largely unavoidable by London’s councils, train operators and the Underground and was soon dealt with. Angus, however, would have the last laugh – for its brief passage through the capital would trigger a series of unexpected issues on the Piccadilly line that are still causing problems today.

To understand why the Piccadilly line is suffering at the moment one must first be prepared to accept two basic facts:

‘Leaves on the line’ is a serious problem Trains can get ‘flat tyres’

Both of these are simple truths, yet for those outside the railway industry they can be surprisingly difficult to accept as fact. This is partly because, for the last thirty years, ‘leaves on the line’ has been the kind of lazy headline that the mainstream press have always delighted in running. It sits alongside ‘the wrong kind of snow’ and ‘trains as hot as cattle cars’ in the grand journalistic tradition of stories that can be quickly thrown out on a slow news day to generate traffic and public reaction.

Those stories provoke a reaction for the same reason that these facts are hard to accept – because they seem to run counter to ‘common sense.’ Both fall foul of what is sometimes known within the tech industry as the ‘Stands to reason test’ – that any phrase you can put the words ‘It stands to reason’ in front of is almost invariably wrong:

It stands to reason that trains are large and heavy, whilst leaves are small and light, so leaves can’t stop a train.

It stands to reason that trains have solid metal wheels, so trains can’t get ‘flat tyres.’

Why leaves matter

As anyone who has slipped on a garden path in the autumn knows, fallen leaves are surprisingly good at reducing friction – particularly when wet. They also have a tendency to render down into mulch. Both of these leaf states are particularly problematic to railway operations.

Trains rely on steel wheels running on steel rails to move. Normally such a combination of smooth-on-smooth surfaces would be highly unwise and lead to slippage, but for trains the benefits outweigh most of the disadvantages – which can be offset by a variety of methods (such as light sanding and surface treatment). The overall weight of the average Underground train also helps ensure wheels adhere to the track.

Leaves are particularly effective at disrupting this relationship between wheel and track, finding their way between the two and causing individual wheels to slip. This is why, on railway embankments and land, coniferous plants and trees are prevalent (when foliage is present at all). For the likes of Network Rail and TfL, deciduous trees are almost as much of an enemy as Japanese Knotweed.

Leaves on the Underground

One could be forgiven for assuming that the ‘underground’ part in ‘London Underground’ would help ensure that leaves aren’t a problem on the Tube. Yet in fact a considerable percentage of the London Underground is actually above ground. Indeed as any veteran London pub quizzer will tell you, only two lines actually stay beneath the surface entirely – the Victoria and Waterloo & City lines.

Those same pub quizzers would likely also be able to tell you that the Central line is the one which spends the most time on the surface – and as a result it suffers its own leaf-related issues every year. What they may not know, however, is that the Piccadilly line is also particularly vulnerable to leaves on the line. This is largely thanks to the presence of the Uxbridge branch, where both public and private landowners outside of TfL’s control seem particularly fond of a deciduous tree line.

Leaf drop on the Uxbridge branch (and beyond) is thus something that London Underground have to manage every year during autumn, as the surrounding trees gradually shed their foliage and this falls or is blown onto the line. To help mitigate this problem the Underground, like most railways, periodically runs ‘Rail Adhesion Trains’ (RATs) which are specially augmented both to help clear leaves and mulch from the track and to carry out other actions, such as light sanding, designed to help reduce wheel slip.

This year, regular RAT runs have taken place on the Piccadilly line as planned using the RAT that is compatible with the line’s layout and signalling.

Unfortunately, it is here that Storm Angus re-enters our story. For this year’s autumn was particularly dry and windless until it arrived. As a result, it appears that overall leaf drop had been relatively low until Angus arrived. The storm, plus the sudden change in weather it partly brought about, changed this situation effectively overnight. The result was an unprecedented level of leaf drop in a short period that seems to have quickly exceeded a level that the Piccadilly line’s mitigation could cope with.

No doubt other factors than Angus will surface during TfL’s own investigation as to what has caused such an unprecedented level of rolling stock disruption on the line. That Angus and leaves have been such a factor, however, seems relatively clear. Certainly sources suggest that shortly after the problems began, the testing of S-Stock on various sections of track shared by the Piccadilly and District lines was swiftly pushed back so that the District line’s RAT could be tested (and cleared) for use on the line instead. This in itself suggests that excess leaf fall was very quickly assessed to be a significant issue.

The form that issue took relates to the second of our ‘stands to reason’ statements – because once track adhesion began to drop, the wheels on the Piccadilly line trains began to slip.

Worn slippy

Wheel slip caused in braking is a major problem for trains because of the effect it has on the wheel itself. When wheel slip happens, the train’s overall weight – so important normally for helping it grip the track – stops being a positive factor and becomes a negative one.

This is because wheel slip rarely occurs at the exact same time on every wheel, on every bogie of the train. Instead there is nearly always more than enough force generated to move the train forward it’s just that one (or more) wheels then either fail to rotate at all or begin rotating slightly later than the rest once traction is regained. In braking, a wheel might stop rotating before the train stops causing the wheel to rub along the track, thus creating a flat section.

When this happens, a single part of the wheel thus stays in contact with the track longer than the rest of it. The weight of the train, combined with the forward motion naturally then causes this section of the wheel to wear quicker than the rest. It doesn’t take much for a wheel to be worn to a state where it doesn’t rotate evenly, keeping its worn section in contact with the track even longer and making the problem even more serious.

And soon, in essence, the train has a ‘flat tyre’.

Why ‘flat tyres’ are a particular problem on the Piccadilly line

Despite the lack of tyres, ‘flats’ are as serious a problem for trains as they are for cars. They seriously affect movement and also cause damage to the track itself. Once a train has picked up a number of flats it thus needs to be removed from service until the problem can be corrected.

On a modern Tube – and indeed rail – line flats are not an unheard of occurrence. Whilst uncommon, they are to a certain extent inevitable. This is thanks to the seasonal problems described above and to the fact that the way trains are driven and braked can also cause flats. Indeed long time readers will remember that the first batch of 378s delivered to London Overground were discovered to be particularly susceptible to flats due to certain issues with their design and had to be packed off back to Bombardier in Derby for fixing.

As a result of the above, most modern rolling stock is both fitted with Wheel Slide Protection (WSP) and designed so that wheels themselves are relatively easy to detach and replace in the depot.

Unfortunately, the Piccadilly line rolling stock is not new rolling stock. Not only this, but (to make things worse) the design and construction of the 1973 Stock (as it is known) came after train design had moved on from traditional block braking (which at least had the benefit of helping to clear leaves and mulch from the wheel) but before modern WSP had been invented.

As a result, the Piccadilly line trains are particularly susceptible to wheel slip when track conditions are bad or if not handled well.

Driving and braking

The difficulty of braking Piccadilly line trains well is likely one of the reasons why some media sources have suggested that bad driving may be a cause for the current issues. Certainly some have attempted to draw a line of cause and effect between the training of additional drivers necessary to man the upcoming launch of the Night Tube on the Piccadilly with the current issues.

Correlation, however, is emphatically not causation. New drivers are trained on all London Underground lines every year, and – when it comes to the Piccadilly – that training also includes tuition (and practice) at cadence braking and defensive driving. According to sources both of these have remained a feature of Night Tube driver training as well.

As on the road, defensive driving is learning to read (and behave) on the road differently in adverse weather to minimise issues. Cadence braking, meanwhile, is a braking technique to be used when surface conditions are poor. On the road this means pumping the brake pedal in order to slow down whilst still maintaining control and reducing skid risk. Mechanically, the technique is different on a train but the end result is the same and – as one might imagine – it is particularly useful on the Piccadilly line given the lack of automated WSP.

TfL’s own report may ultimately thus show that driver behaviour played a factor, but it seems unlikely. Instead the two biggest causes for why the problem occurred will likely be much more prosaic – the wrong type of weather hit the wrong line, with the wrong type of rolling stock at the wrong time.

Wear and lathing

Why the problem has persisted, and why it is taking so long to recover, is also largely due to simple logistics.

When a train has a flat there are two ways it can be fixed. Firstly, by replacing the wheel with a brand new one. Secondly, by turning it on a wheel lathe until it is round again. The latter is possible in situations where the flat isn’t too severe, the rules (generally) being that a wheel can be reduced by up to 10% of its diameter before it is no longer fit for service.

Whether new or turned, it is not simply a case of replacing the wheel, however. Whilst wheels can vary across a whole train by up to 10% overall, for obvious reasons of stability and balance they can’t vary that much in close proximity to each other.

As a result, even changing one wheel requires work and perhaps replacement of the adjacent and opposite ones as well. On the 1973 stock, and indeed most Underground stock, both wheels on an axle must have almost exactly the same diameter. Beyond that all the wheels on a single bogie must have a very similar diameter and finally all the wheels on the train car or carriage must be broadly similar.

Changing a single wheel, therefore, may trigger an extensive rebalancing exercise across the entire train – and also require the adjustment of everything from tripcock position to suspension height before it can be completed.

It is work like the above that the Piccadilly line depot staff have been working hard to complete. Logistically, it is a situation made worse by the fact that the only wheel lathe on the line is at Northfields’ Depot and using it renders a whole maintenance pit unavailable for wheel replacement work. In previous years London Underground have been able to temporarily hire a mobile lathe, but for reasons not yet clear (but likely related simply to availability) this hasn’t happened this year. TfL have indicated, however, that where possible Piccadilly line trains have now been making use of lathes they can get to elsewhere on the network.

Even with this, however, wheel replacement is a slow process and one that simply can’t be accelerated as it’s a problem of capacity, not effort.

An unusual confluence of events

All of the above hopefully goes some way to explaining why the Piccadilly has struggled so much in recent weeks. As human beings we like to try and ascribe a single cause to events – one single problem or person who is to blame. In part this is because we live in an era of simplified narrative, but it is also because a single issue implies a simple – and hopefully speedy – solution.

Sadly none of that is true on the Piccadilly. TfL’s own investigation will no doubt reveal more about what happened and why, and may even highlight some extra causes and effects. At the most basic level though the issues on the Piccadilly, like many issues that affect the railway, are simply the result of a number of minor factors combining to form a greater problem.

Unusual weather has resulted in a greater-than-usual risk of flats, something the Piccadilly line trains are unfortunately more susceptible to than modern rolling stock. The more flats created, the more trains are withdrawn from service.

Unfortunately there are hard, physical limits to how many trains can be serviced on the Piccadilly line at a time. Once these were hit, the number of trains in service began to drop, forcing cancellations and increasing pressure on the trains still in service a number of which then developed problems themselves.

All of this, from a commuter perspective, has been further aggravated by the fact that one branch of the Piccadilly line was not only directly aggravating the problem but is arguably lower priority than the other (which links London to Heathrow). For both of these reasons the service pattern that has remained in force has generally favoured Heathrow – a major inconvenience to regular travellers on the other branch.

Things can only get better?

Whilst it will come as no comfort to those currently affected, the truth is that the only thing which will restore full service to the Piccadilly line is time. There are no magic means by which the backlog of repairs can be speeded up and whilst TfL’s report will help highlight ways this situation can be avoided and further mitigated (if possible) in future that will not help travellers now.

What remains to be seen is whether this month’s planned launch of the Night Tube on the line will also go ahead. No doubt TfL are under pressure from the Mayor to see that deadline met, but at the same time neither party will want to do so if there’s a risk that it will damage day time services even further.

Perhaps this, then, is the barometer which affected passengers should look to for now. If the Night Tube launches then this would suggest that the worst of the damage has been overcome. Should it be delayed, then that may suggest that there is still some way to go yet.

Cover image by Romazur