The much-troubled project to re-signal the four lines comprising the Sub Surface Railway (SSR) is once again in a committed contract situation. This is for the third time of asking but many lessons have been learned along the way, both by London Underground and the various supply companies who have been involved.

The four lines, with their colours on the London Underground map, are:

– Metropolitan (magenta)

-Hammersmith & City (pink)

– District (green)

– Circle (yellow)

None of these are self-contained and all have interfaces with each other as well as with other LU lines and some Network Rail routes. It is thus a complex piece of railway and failing to understand the operation has led to some of the past problems.

At the time of the PPP (Public Private Partnership) initiative, the lines were part of the Metronet grouping and the plan was to equip the lines with the then Invensys DTG (Distance to Go) radio system, which later on was successfully deployed on the upgraded Victoria line.

With Metronet succumbing to political and contractual pressures, the Invensys (now Siemens) system intention was abandoned.

A subsequent contract was awarded in 2011 to Bombardier employing its CityFlo product, successfully used in Madrid and Shenzhen Metros and considered to be a mature technology. The contract value was £364 million, a very competitive price. However, problems over the interpretation of technical requirements, timescale and cost soon emerged and eventually, in 2013, the contract was terminated by mutual consent.

By this time, new trains for the SSR were well into production so any chance of fitting the new signalling equipment in the factory was lost. In parallel, LU had equipped firstly the Jubilee line and more recently the Northern line with the Thales Seltrac control system and had learned some hard lessons on how its approach to a modern CBTC (Communications Based Train Control) system needed to change. A full description of the Northern line upgrade was given in the Rail Engineer May 2015 edition.

With these systems now successfully in operation, a new contract for the SSR re-signalling was eventually awarded to Thales in July 2015 at a value of £760 million, having been preferred bidder for some time before that. This is more than double the earlier contract so requires some explanation.

A recap of Seltrac

Originally designed in Canada and firstly used on the Vancouver Skytrain, Seltrac has been deployed on many other metros around the world, including in the UK – the Docklands Light Railway and the two LU lines referred to. It is a fully-fledged combined ATP and ATO package that conforms to the SIL4 requirements for operational safe software. It also has moving block capability, allowing trains to close up when busy periods are encountered.

The components of the system are:

» The VCC (Vehicle Control Computer) at the control centre to manage and control all train movements;

» An SMC (System Management Centre) acting as the man-machine interface;

» An SCS (Station Controller sub-station) at each stopping point for station information updates;

» A VOBC (Vital On Board Computer) on each train to control all train movement and braking commands.

All of this equipment is fully duplicated, the VOBCs being positioned at both ends of the train. Much of it will be identical to the equipment installed on the Northern line. The Seltrac system has seen various upgrades since its initial design, the VCC now being a third-generation product.

The Jubilee and Northern lines used a loop-based system, these being mounted between the running rails with a transition every 25 metres to give a positional reference point. However, Thales is mindful that other CBTC suppliers have moved to radio for the transmission media, this being less intrusive in terms of infrastructure disturbance, and has developed a radio-based alternative that is already in service in South Korea and China.

Therefore, whilst the SSR lines will have the same operational methodology as the earlier projects, there will be differences in the technology that will need to be tested thoroughly.

Contract progression

The Bombardier contract had made some progress before being abandoned, with the new control centre at Hammersmith constructed and some SER (Station Equipment Rooms) made ready, all of which will be reused. With Thales as preferred bidder, agreement was reached that concept design work could proceed. This was finished by the end of 2014 and the preliminary design, comprising system requirements and interface specifications, will be completed by the end of 2015. The final design including the application of the system to the existing railway is a 2016 activity.

In parallel, installation work can proceed from early next year with Thales using various subcontractors for the necessary trackside work. More importantly, testing of the system comprising a full wayside installation and prototype equipment on a test train at the Old Dalby test site is already underway. Modifications may then be required and a second train with a finalised train configuration will be ready by summer 2016. This represents the ultimate confidence builder, both for client and supplier, and will lead to the provisional roll out stages of:

  • 2018/19 – Hammersmith to Paddington, then the rest of the Circle line;
  • 2019/20 – Continuance to Upminster and to Wimbledon, Richmond and Ealing;
  • 2019/20 – Northwards from Baker Street to Uxbridge, Watford and Amersham.

These dates are aspirations and better the contract requirements. It remains to be seen whether they can be achieved but hopes are high. It might seem strange that the work progresses from the centre outwards but this takes account of not having to deal with the complication of interfacing with other lines until the system operation is understood and bedded in. It also enables the biggest benefits in terms of capacity gain to be obtained as early as possible including better regulation of the flat junctions at Edgware Road, Baker Street, Aldgate and Earls Court/Gloucester Road.

Technical aspects

The operation of the Seltrac system will be essentially similar to that already in service on the Jubilee and Northern lines. Trains receive a communication from the control every second to ensure continuance of the Movement Authority but, if no message is received within five seconds, an emergency brake application will occur.

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Train positioning data is derived from track mounted balises (TAGs) positioned in a multiple of 25 metres, these being the equivalent to the loop transition points. Once passed, the train’s odometer counts the distance until the next TAG. In busy areas, and at the approach to junctions or stations where a train is braking, the spacing will be 25 metres. In country areas, distances may not be so critical and TAGs may be as far apart as 200 metres.

The TAGs are locally powered by a battery with a 10 year life but incorporating an alarm when power is running low. Should one fail or be misread, it is not serious as an update position is obtained from the next TAG. Axle counters will be used as a backup when degraded mode is in operation, these being positioned near stations and critical junctions so as to form logical block sections.

Trains communicate the latest TAG information to the VCC, which is continually scanning ahead for other trains, stations and point positions and will call for routes to be set as appropriate. The movement authority is regularly updated to tell a train how far it can go and at what speed. A safe separation distance of around 100 metres between trains is achievable but will depend on gradients and braking profile conditions.

Normally trains will operate in ATO mode but drivers can switch to protected manual mode with permission. Such occasions may come about during unusual adhesion conditions or when engineering works mean trackside workers being present.

The introduction of radio transmission will be a learning curve. No dedicated radio channels are envisaged, instead use will be made of the unlicensed 2.4GHz public Wi-Fi band. Frequency-hopping using algorithms to search around the band and a security protocol to an international industry standard will be employed. Free space propagation will be used throughout, even in the tunnels, and no use of radiating cable is foreseen.

The aerials will be positioned approximately every 250 metres, mounted on a six- metre mast for above ground lines and on the side wall of tunnels about half way up. In twin track tunnels, alternate aerials will be on opposite sides of the tunnel so as to minimise the blocking effect of one train to another coming in the opposite direction. All of this will ensure that the loss of any one base station will not adversely impact on coverage to the trains.

A full radio survey on the central section has already taken place during ‘engineering hours’ using kit mounted on two trolleys to replicate a train and portable base stations in the predicted positions. Further surveys on other sections of SSR are planned as the project progresses. Full data recordings have been made of the radio signals such that installation of the network infrastructure can now proceed.

The radio system will be fully duplicated to achieve maximum redundancy borne upon a new fibre cable network designed to form overlapping transmission rings. Power supplies will also be duplicated. The new cables will have spare fibres that might be used for other operational systems or, indeed, by third parties.

Junction optimisation will be key to achieve the required 32 trains per hour in the central section. Normal operation will be to timetable mode but, where trains are out of sequence, then a ‘first come, first served’ with caveats will kick in. The system will know if any trains are ‘fast’ over a section (particularly true of the Metropolitan line), so allowing a slow train to proceed ahead should not happen. Equally important is not allowing too many trains to approach a restricted terminus, such as Aldgate, if platform capacity does not exist.

Fitting the trains

By the time of first commissioning, all the S-Stock trains will be in service. The work to fit the trains with Seltrac ATP, ATO and radio equipment will be by returning the trains to the Bombardier works at Derby for this to be done. Most equipment can be contained within the driving cab but under floor tag readers, roof aerials, odometry and power supply elements need to be interfaced into existing components.

The engineering fleet including ‘yellow plant’ will also need to be fitted, but this is being planned as an in house job.

Interworking with other lines

Several sections of the SSR share services with other lines and train operators. These represent a challenge as to how the Seltrac system interfaces with other signalling systems. There is no single solution and each one has to be considered individually.

» Rayners Lane to Uxbridge shared with the Piccadilly line – Seltrac will be fitted for SSR train running with fixed block signals retained for Piccadilly line trains.

» East Putney to Wimbledon – the Seltrac will be overlaid on the Network Rail signalling controlled from Wimbledon Power Box with lineside signals retained and movement authorities governed by the signal sections.

» Gunnersbury to Richmond – tracks are shared with London Overground so conventional signalling has to be retained with Seltrac movement authorities aligned to that.

» Barons Court to Acton Town – District and Piccadilly line trains can share tracks between Chiswick Park and Acton Town.

» Harrow on the Hill to Amersham – tracks owned by LU but Chiltern Railways trains have running rights for their Aylesbury service. The existing LU signalling will be replaced with standard Network Rail three- aspect signals controlled by the Seltrac system intelligence such that ATO operation can be maintained for SSR stock. This is to be known as an ‘underlay’ and LU drivers will see a blue signal under normal circumstances. The train description system will know which trains are controlled by ATO and which are manually driven. Chiltern line drivers will continue to see either a red, yellow or green aspect and will drive accordingly. Their trains will retain trip cock apparatus to trigger any signal overruns. It will not be permitted for a standard red, yellow or green aspect to be displayed at the same time a blue light is showing.

»  Watford High Street to Watford Junction – will only be required once the Croxley Link is constructed but, on the current programme, this will be completed before the Seltrac system reaches the extremities of the Metropolitan line, hence the new section will require initially to be fitted with conventional LU signalling. LU is still considering whether to subsequently extend the Seltrac CBTC into Watford Junction but the signalling on the section from Watford High Street to Watford Junction shared with London Overground trains will need to be decided.

All of these represent both a technical and commercial interface challenge which will require sensitive and careful negotiations with the various parties involved.

Other factors

The basic track layout will remain unchanged, although there will be some faster turnouts to allow faster run-in times. Little-used junctions will be plain-lined or moved to somewhere that is operationally beneficial. To install, test and commission the new equipment, some 70 partial closures at weekends will be required. The Connect track-to-train Tetra radio system will remain, as will the CCTV surveillance of platforms but with the driver’s viewing screen now incorporated into the S-Stock cab. The drivers remain in control of door operation and have overall supervision of the ATO operation. This includes an emergency stop button should anything untoward be observed at the lineside.

Consultation on the new operation with the trade unions will be based upon the Northern line experience that is essentially similar. Training will be mainly by cab simulators but a section of the Hammersmith line will also be made available for driver familiarisation of the actual system.

Key to success will be the housing of the joint LU/Thales team in a common office that worked so well for the Northern line project. With all that has gone before, it is a near certainty that this time the project will progress to a successful conclusion. Further updates will be featured in Rail Engineer as the work continues.

Thanks are expressed to Andy Bourne and Stuart Harvey, respectively the project manager and programme manager for LU, and Andrew Hunter and Andy Bell, the chief engineer and programme manager for Thales, for their time and patience in explaining the system. Thanks also to Sandeep Dhillon from LU and Gayna Hall from Thales for facilitating the meetings.