HomeIndustry NewsUpgrading London Underground’s Central line tube stock

Upgrading London Underground’s Central line tube stock

Listen to this article

The demand for train services in London is large, growing, and is expected to continue to do so despite the hiatus caused by Covid-19. This trend has been apparent since the early 1980s. London Underground (LU) has been running more and more services for over 30 years and has been upgrading its lines’ capability as trains and signalling become due for renewal. The Central line (see Panel) was the first such integrated upgrade with the 1992 tube stock trains introduced from 1993 and ATO in 2000/2001.

The trains are now nearly 30 years old and are doing far more work than was ever planned, some 160,000 km per train, per year compared with the designed 127,000 km. This article will describe how London Underground is dealing with train reliability and obsolescence issues for the next 10 years or so.

During the early 1980’s LU started to introduce electronics on its trains and there was also great interest in further reducing train weight. 1973 tube stock (Piccadilly line), D stock (District line), and 1983 tube stock (Jubilee line) were fitted with electronic equipment monitoring systems, and LU had pioneered the use of aluminium in the construction of its trains. Tube trains had retained the steel underframe, but LU wanted to take advantage of the strength and light weight of monocoque aluminium structures using large aluminium extrusions. In addition, GTO thyristor ‘chopper’ control and electronic control systems were becoming readily available.

Chopper control allowed much higher performance in the open sections of the line whilst enabling lower speed in tunnels without the waste of energy that would have been involved if resistance controllers had been retained, as there were no resistors needed in the motor circuits for lower tunnel speeds. This and ATO delivered a much-improved service frequency with reduced journey times. 85 trains were required, only about five more than the old fleet size. All these concepts were tried out on the 1986 tube stock prototypes prior to the 1992 tube stock being ordered from British Rail Engineering Ltd in 1989. 1992 tube stock is made up of four, two-car units with all axles motored. They have a lot of electronics and are the lightest passenger cars on the LU network.

Most of the electronic components specified were new and had never been fitted to trains before. They included components that could reasonably be expected to be obsolete long before the end of the nominal 40-year life of the mechanical parts. No one knew what the life of the electronics might be, but the best advice was ‘about 20 years’.

In service, the 1992 tube stock has been more troublesome than had been hoped and has required significant unexpected expenditure to keep it going, including the replacement of the bogie frames due to premature cracking. The traction motors themselves have a very hard life with frequent motoring and braking and need a lot of maintenance – much more than forecast. Given that there are 32 motors on every train, this has been a significant burden. There have also been some body cracks. The electronics have been reasonably trouble free but, over time, some of the individual components and, indeed, solder and printed circuit cards have started to fail. Something had to be done, leading to a programme of works known as the Central Line Improvement Project (CLIP), valued at approximately £500 million.

As well as dealing with reliability and obsolescence issues, the programme also includes provision of features to comply with the Rail Vehicle Accessibility Regulations, age related body repairs, and some heavy maintenance activities. Apart from RVAR legal compliance, the project is expected to deliver reduced maintenance cost, a significant increase in reliability and, as a bonus, there is also an expected 120 second reduction in journey time from end to end. Rail Engineer visited London Underground’s Acton Works in November 2022 to see the work in progress.

The scope includes:

  • Replacement of the DC chopper system and DC motors with a modern three phase AC traction package. This involves replacing 2,720 traction motors, fitting 680 variable voltage variable frequency inverters, and replacing 340 auxiliary converters. Supplier: Alstom Sweden, motors from ABB Sweden.
  • Replacement of the Train Management System (known as DTS) that will help support the passenger information system capturing more data from upgraded systems with automatic download via 4G and station Wi-Fi. Supplier: EKE-Electronics, Finland.
  • Upgrade of the passenger information system with in-saloon displays. Supplier: EKE-Electronics/FOCON.
  • Install saloon security CCTV using two cameras per car.
  • LED lamps requiring less power (to enable PIS and CCTV) and delivering better illumination. Supplier: LPA, UK.
  • Modification to provide compliance with the Rail Vehicle Accessibility Regulations 2010, including spaces for wheelchairs to a design by LU.
  • Repairing door pillar cracks and floor corrosion on all cars; typically 80-90 welds per two car unit.
  • Completing replacement of body ends originally started in 2012.
  • Door overhaul (8160 pneumatic door engines and doors).
  • Bogie overhaul (1340 bogies).

The first trains are currently being modified. The first is due in service in March 2023 and the programme is due for completion in 2027. Observant readers will see that this date is well beyond the 2020 deadline for RVAR works; LU has obtained a dispensation from the DfT.

Planning and programme
Talking to Project Manager Sam McDonough it was clear that this extensive programme would have been even more complicated had trains been sent to established major modification centres around the country as moving tube trains over the main line network is far from trivial. In the end, LU decided to do the work in house and in its plans, LU also anticipated the need to overhaul the large sub-surface S stock fleet and forthcoming Piccadilly line 2024 tube stock both of which are not easy to separate into individual cars.

To satisfy this long-term need, a new 170-metre-long workshop known as AC14 has been constructed on former sidings at Acton Works. This provides four 150-metre-long pitted roads, (two of which are each equipped with 32 Mechan lifting stations enabling simultaneous lifting of an 8-car train or individual units/cars), as well as a great deal of storage and office space. For CLIP, AC14 is used for ‘clean’ work, and the adjacent shop AC15 (formerly known as ‘Heavy Repair’) for ‘dirty’ work.

Sam outlined progress since the project procurement kicked off in 2018 and physical work in 2019. Train 1 was the prototype for the AC traction conversion. This work was carried out by Bombardier (prior to Alstom’s take over) in Derby with testing at RIDC Melton. Train 2 was used to develop the saloon electronic equipment installation and Train 3 is the first train with all the modifications. Train 4 will be the first to include bogie overhaul.

When the programme is up to speed, there will be five trains out of service and each will take 10 weeks/50 days to complete, with one train out of service/back into service every 10 days. To deliver the five-train float, the service requirement was reduced by two trains and the remaining three came from the maintenance float. Sam said that, with declining reliability, fleet availability was “tight”.

Each train goes though these stages:

  • Ruislip depot: Train fitted with tripcock and driven via the Metropolitan and Piccadilly lines to Acton.
  • Acton Works AC15: Train split into individual cars and moved via traverser to AC15. Doors and door engines are removed for overhaul; DC chopper and other redundant material removed for scrap; seats removed; floor coverings and door pillar covers removed to allow inspection and rectification of cracks/corrosion; fittings removed from centre of cars 2 and 7 to allow fitting of RVAR wheelchair bays; body ends replaced where required; new brackets fitted; and all drilling carried out.
  • Acton Works AC14: Cars coupled into units. Doors and door engines refitted; extensive new wiring installed; brackets to support new saloon displays and window masks fitted; new lights, CCTV, and TDS fitted; traction equipment, brake resistor, and static converter fitted; cars lowered onto bogies fitted with new traction motors; recovered seats fitted; static testing.
  • Acton Works: Train tested on short test track; driven back to Ruislip depot.
  • Ruislip depot: Tripcocks removed; ATO/ATP recommissioning; authorised to enter service.

Examining trains 1, 2, and 3 in the workshop, it was clear that accommodating all the required equipment in the small space of a tube train is a major challenge with the CLIP requiring more equipment and cabling, not less. Engineers described some of the clever repurposing of existing cables to suit the new LED lighting but, in general, space had to be found for more cables, often in the ceilings where space is severely constrained. There are six seat bays on each car, each of which accommodates two door engines, a heating and ventilation system with much corrugated ducting as well as a variety of equipment.

As Emma Costerton, CLIP’s Electrical Engineer, said: “Every seat bay became a mini project to determine equipment layout and wiring.” Given the number of different car types, that’s approximately 20 different seat bays and in each one the new equipment had to be located and additional wiring provided, whilst still allowing space for access to and replacement of door engines.

With the new lighting, new flooring and new seats, the trains are starting to look good again and Sam advised that some ‘vinyl; film decoration will be carried out. He added that some 100 people are currently employed on CLIP physical works and this number will ramp up to about 180 when the project is completely up to speed.

What next?
The remaining life of the 1992 tube stock is still a live issue. It was bought with a nominal life of 40 years which, based on entry into service, suggests nominal life expiry between 2033-35. But the nominal life of the existing Bakerloo and Piccadilly line trains was circa 36 years, and they are already well into their 40s. When the contract with Siemens for new Piccadilly line trains was announced reference was made to options for Central line (and Bakerloo line) trains. But since then, TfL’s financial situation, particularly following Covid, has changed significantly. It is possible therefore that 1992 tube stock will have to soldier on, with further engineering interventions from time to time as other parts become obsolete.

Thanks are due to Sam McDonough and the CLIP engineering team for their help in preparing this article.

Central line
The Central London Railway opened in 1900 as the third of London’s tube gauge railways. It originally ran from Shepherd’s Bush to Bank and, with gradual extensions, it eventually reached Ongar (cut back to Epping in 1994) and Hainault in the east and Ealing Broadway and West Ruislip in the west. The line serves 49 stations and is approximately 74km long.

It was originally built with a smaller than standard tunnel diameter, centre third rail, and locomotive hauled trains. It has undergone many transformations and changes of rolling stock over the years, such that it can now be served by standard sized tube trains but with high lift shoe gear. It does have two significant constraints in the form of the most severely curved platform at Bank and also a very small-radius curve between Shepherd’s Bush and White City.

Its designers provided one of the first energy saving features in the form of hump profiles. The track descends on the way out of stations and rises on the approach thus using gravity to help accelerate and brake the trains. This feature proved to be a constraint when the platforms were extended in the 1930s and visitors to central London platforms can observe that platform ends are sometimes on a gradient.

Pre-Covid, it was the busiest line on the Underground carrying over 250 million journeys annually. It serves four of the 10 busiest stations and also serves five of the 10 least busy stations on the entire network. Passenger numbers are expected to grow significantly before the line is next due for upgrade, despite what is believed to be temporary relief brought by the Elizabeth line.
The current peak service frequency is between 27 trains per hour (tph) and 30 tph. Off peak the service is 24tph.

Malcolm Dobell BTech CEng FIMechE
Malcolm Dobell BTech CEng FIMechEhttp://therailengineer.com
SPECIALIST AREAS Rolling stock, depots, systems integration, fleet operations. Malcolm Dobell worked for the whole of his 45-year career with London Underground. He entered the Apprentice Training Centre in Acton Works in 1969 as an engineering trainee, taking a thin sandwich course at Brunel University, graduating with an honours degree in 1973. He then worked as part of the team supervising the designs of all the various items of auxiliary equipment for new trains, which gave him experience in a broad range of disciplines. Later, he became project manager for the Jubilee Line’s first fleet of new trains (displaced when the extension came along), and then helped set up the train refurbishment programme of the 90s, before being appointed Professional Head of Rolling stock in 1997. Malcolm retired as Head of Train Systems Engineering in 2014 following a career during which he had a role in the design of all the passenger trains currently in service - even the oldest - and, particularly, bringing the upgraded Victoria line (rolling stock and signalling) into service. He is a non-executive director of CPC Systems, a systems engineering company that helps train operators improve their performance. A former IMechE Railway Division Chairman and a current board member, he also helps to organise and judge the annual Railway Challenge and is the chair of trustees for a multi academy trust in Milton Keynes.


Please enter your comment!
Please enter your name here

This site uses Akismet to reduce spam. Learn how your comment data is processed.