London’s first tube railway was the 1891 City & South London line. Trains were hauled by electric locomotives, some of which were built by Siemens Brothers. The tunnels were even smaller than today’s, circa 4m in diameter. The Tube has expanded significantly and now requires more than 540 trains to maintain the service. It is characterised by small tunnels leading to a constant search for ways to make the best use of the limited space.
There have been just four generations of tube trains. The first locomotive-hauled trains were built in the late 19th/early 20th century. Next, motor cars with electrical compartments behind the drivers’ cabs were developed and built until the mid-1930s. It was in 1935 that the London Passenger Transport Board led the development of prototype trains with all the equipment under the floor, leaving more space for passengers. These had bi-parting doors towards the middle of the cars and single doors at the ends. The prototypes led to the 1938 tube stock, one example of which is still owned by the London Transport Museum and used occasionally on the network. This configuration was built in six, seven and eight-car formations for over 70 years, culminating in the 2009 Victoria line stock. So what’s next?
In November 2018, London Underground (LU) placed a contract with Siemens Mobility for a fleet of 94 trains for the Piccadilly Line, with options covering further expansion of the fleet and to replace the Bakerloo, Central and Waterloo & City lines’ trains. Nothing more was published until a joint Siemens/London Underground press conference in March 2021.
This article gives some background to LU’s thinking and an outline of Siemens’ design.
By 2003, the configuration of the large-profile S-stock had mostly been settled, featuring air conditioning, walk-though wide gangways and all double doors. But how could these features be applied to tube-profile trains?
In the late 1990s, LU had carried out a study aimed at maximising tube train capacity, christened the Space Train. Using this work, it started thinking about how S-stock’s features might be incorporated into the next tube stock. One view was ‘specify what you want and industry will deliver’, but it is not that simple due to the small size; these are bespoke products presenting many challenges.
Double doors: the space available is so small that floor level is below the top of the wheels; this means there can be no doors above the bogies. The typical layout of two double doors towards the middle and two single doors at the ends of the cars was developed in the late-1920s. If the end doors were eliminated, there would not be enough doors.
Through gangways: these must have enough flexibility to accommodate horizontal and vertical curves as well as relative movement between vehicles. The bigger the overhang beyond the bogie, the larger the relative movement between vehicles and the longer the gangway. Having the single doors means long overhangs beyond the bogies; as a result, delivering both single end doors and gangways is virtually impossible.
Air conditioning: it has long been assumed that air conditioning produced too much heat to be dealt with in the small tube tunnels. Even if this issue were to be solved, there was no space to accommodate the equipment.
And the solutions
Starting with space first. Bogies take up most space under trains, so could there be a solution that used fewer bogies? Articulation was the obvious answer, but this means that each car body must be shorter as there was little or no scope to increase the 10-11m bogie spacing. This meant that more cars were required, but there would still be fewer bogies providing an extra length of about 9m for equipment and about 5t in weight saved per bogie eliminated. End single doors would be eliminated, replaced by more double doors on the additional cars. Articulated layouts make the gangway shorter because the movements that have to be accommodated are smaller and limited to rotational movements around the articulation centre.
Easy? No! LU’s early work showed that the typical articulation where the centre of the bogie is under the joint between the vehicles is not suitable for tube trains. To fit a bogie with the vehicle-vehicle coupling above it, the floor of the gangway above that and still achieve enough headroom in the gangway would be extremely difficult.
Also, articulated bogies (known as Jacobs bogies) often have a longer wheelbase than their non-articulated siblings. A longer wheelbase makes it hard to improve curving/track-friendliness and the opposite of the ambition to reduce the wheelbase below the usual 1.9m. These issues led to a proposal where the bogie was wholly under one end of a vehicle and the other end would be supported from the bogie end of the next car. It was inspired by the Stockholm Metro C20 trains and is the configuration adopted for the new Glasgow subway trains.
The extra underframe space released by articulation helps to accommodate the air conditioning equipment. Dealing with the heat is more of an issue. One helpful factor is the overall reduction of energy needed on a modern train with regenerative braking and where the power system and train are designed as a system.
That said, with an eventual 50% increase in train frequency from 24 trains per hour to 36 (with new signalling), the overall power requirement – and therefore heat – will increase irrespective of the air conditioning. Some platform and tunnel ventilation and cooling schemes are likely to be needed to accommodate the increased frequency and any additional ventilation requirement for air conditioning would be a marginal extra cost.
By 2010, LU was confident in its development work and formed a team to take the project forward. After contract award in 2018, covering options for almost 250 trains, Siemens committed to a factory in Goole, East Yorkshire, and to build at least 50% of the Piccadilly Line trains there. Since 2018, Siemens and LU have worked together to compete the trains’ detailed design. Construction of the factory has begun and the first apprentices recruited.
At the press event in March 2021 – which included a recorded message from the Mayor of London, Sadiq Khan – LU’s Managing Director, Andy Lord, and Siemens Mobility CEO, Michael Peter, launched the Inspiro London train design and described the key customer-facing features.
Subsequently, Rail Engineer interviewed Dave Hooper, Siemens Mobility’s Director of Major Programmes, to find out more. Engineers from the UK, Germany and Austria have worked together over the last year to complete the detailed design without any face-to-face meetings as a result of the Covid-19 pandemic.
Dave Hooper couldn’t praise the joint teams highly enough. He added that the experience has led to a completely new way of working which will persist once the pandemic has ended, blended with face-to-face meetings where necessary to add further value. Dave thought the team working was absolutely vital on this project.
He had been programme director for the Thameslink Class 700 trains where a ‘platform’ design was customised for the Thameslink application, whereas the LU train is unique with a bespoke design for this customer alone and it makes perfect sense to work with the customer’s engineers so that Siemens may thoroughly understand the subtleties of the LU environment.
The Piccadilly Line train will be a nine-car, ten-bogie articulated train which will be 113.7m long, 2.844m high and 2.648m wide over the external sliding doors. Dave explained that Siemens came up with an innovative solution, having evaluated many options. It will be formed from five two-bogie motor cars with four intermediate cars – with no wheels – supported between adjacent cars. As far as your writer is aware, this is a unique configuration for a metro train, although it is relatively commonplace in tram and light rail vehicles.
Eight bogies will be motored, with the trailer bogies located under the cabs. The articulated couplings below floor level will accommodate yaw, pitch and roll, and one of the two couplings on each intermediate car will be supplemented by a device at roof level to control roll. The bodies will be formed of welded aluminium extrusions.
And the bogies
The bogies will be inside frame type, with a wheelbase of 1.80m using radial arm wheelset guidance supported with coil spring primary suspension. The radial arm’s bushes will be of variable stiffness to allow a modest amount of passive radial steering. The secondary suspension will use rubber hour-glass-shaped springs, previously used on several LU fleets. Hydraulic dampers will be used on both suspension stages. There will be single tread-brake units per wheel for brake blending and backup in case of dynamic brake failure and for emergency brake applications. There will be space for shoes, tripcocks, sleet brushes, wheel flange and wheel tread lubrication, and CBTC antennae.
Although the individual axle loads will be higher than usual due to the articulated layout, modelling has shown that the bogie’s running stability will be improved, making it much more track friendly than any previous LU bogie. Siemens is also working with LU to develop an active steering version which might be required on the Bakerloo and Central lines.
The 16 motors per train will be the three-phase permanent magnet type, driving the axles through double-reduction gearboxes. The motor and gearbox will be highly integrated and fully suspended on the bogie, driving the axles through a flexible coupling. Each motor will be driven by its own inverter and there will be no unprotected line voltage cables carried between cars. All bogies – except those on the central motor car – will have electrical power collector shoes which will be connected to a high-voltage (HV) box containing the main circuit breakers. The power is then fed to two traction converter boxes on the adjacent intermediate car.
A converter box contains two inverters and braking resistors, each feeding a traction motor on the adjacent bogie car. As an example, based on cars numbered 1 to 9, the shoes on Car 1 feed the traction converter boxes on Car 2 via the HV box. The inverters in one converter box feed the adjacent motor bogie on Car 1 and those in the other converter feed the adjacent motor bogie on Car 3. Similarly, shoes on Car 3 feed the converters on Car 4 which feed the motors on the adjacent bogies on Car 3 and Car 5. In this respect the train is symmetrical about the centre of Car 5.
Regenerative braking will generate up to 980V. Individual control of the motors allows very flexible control of dynamic braking in poor adhesion conditions compared with previous generations of AC drives where several motors are connected in parallel to the inverter, leading to the dynamic braking possibly being substituted by friction braking in poor adhesion conditions. The maximum traction power will be 2.5MW and the top speed 100kph.
The auxiliary power will be provided by static converters using silicon carbide technology with lithium-ion batteries and all lighting will use LED lamps. The three auxiliary power supply modules will each deliver 400V three-phase AC rated at 90kVA and 110V DC rated at 40kW. Through light weight, regenerative braking and low-energy auxiliaries, Siemens claims that the new train will use 20% less energy compared with the old train after allowing for the power consumed by the air conditioning system.
The battery is primarily intended to provide power to the auxiliaries, but it can also move the train over a limited distance. This will be useful in depots – to move the train from the maintenance areas onto the conductor rails – and recover trains in tunnels between platforms in the event of power failure.
Packaging the small underframes on tube stock is always a challenge and the layout has to reflect both the functions required and an even weight balance.
There will be a total of 18 passenger doorways per side, plus cab doors. The passenger doorways will be 1690mm wide, compared with 1446mm on the current Piccadilly stock. There will be a multi-purpose area with tip-up seats in every car and four designated wheelchair spaces per train. Overall there will be room for 260 seats – including tip-ups – and 808 standing spaces at 5 passengers/m2; a total of 1,068 passenger spaces, an increase of over 20% on the old train. The doors will be electrically powered, with door operators above the doors for the first time on a production tube train.
The exterior and interior appearance is based on work for LU by Priestman Goode and is what might be described as ‘a modern take on the traditional LU heritage design’, with appropriate use of Piccadilly Line blue throughout. Passengers will find it easier to access the train and move around due to larger door openings and walk-through carriages. Floor height at doorway thresholds will be 700mm which is higher than the standard 520mm platform height; LU will be providing ‘humps’ next to designated wheelchair doorways in accordance with usual practice. The other key customer benefit will be the air conditioning.
Passengers will also have digital information screens displaying real-time information as well as variable or even video adverts: no more staring at the same product for the whole journey! Dave Hooper also assured Rail Engineer that the seats will be cushioned and not ‘metro train’ plastic.
Reliability and depots
LU set some tough reliability and availability targets, and Siemens said these will be delivered using redundant system design of vital components. There are also tough lifecycle cost targets and Siemens is providing its Railigent® smart remote monitoring system to support optimising maintenance requirements and fault finding. The aim is to detect anomalies remotely and get the train back to the depot in a planned way before a fault occurs. There will be a comprehensive train control and monitoring system which will also collect data about the performance of the trains’ sub-systems and allow that data to be downloaded remotely via Wi-Fi. Finally, the train has provision for ATO/ATP equipment and retains provision for driverless operation at some point in the future.
Naturally the depots will need work. Northfields and Cockfosters will be upgraded to accommodate the longer trains and provide facilities for dealing with the different configuration of underframe equipment. This will take account of the fact that articulated through-gangway trains are much harder to separate into individual vehicles than the older, conventional trains. Dave Hooper observed that cars with no wheels are difficult to handle once uncoupled. London Underground is managing the depot upgrades with input from Siemens.
Siemens and LU said that they expect the first new train to go into service in 2025. They are also optimistic that, by then, funds will have been made available to order the trains for the Bakerloo, Central and Waterloo & City lines. Rail Engineer wishes the joint project team every success and looks forward to seeing the trains for real in due course.
Finally, it is worth noting that the basic design would be the same for the Bakerloo Line – nine-car trains; the Central Line will have 11 cars and the Waterloo & City five cars. Car lengths will be adjusted for the Central line trains to conform to that line’s train length limits.
Thanks to Katie Byrnes and Laurie Waugh from Siemens, and Nancy Ryder and Claire Jermany from TfL for their assistance with this article.