Much has been written to explain the operation and benefits of Communications Based Train Control (CBTC). By default, such technology has become associated with modern metro operations around the world. Lines that employ CBTC enjoy a significant increase in train running capacity as well as achieving automatic train operation (ATO), automatic train protection (ATP) and improved information to the travelling public.
However, converting an existing line from conventional signalling to CBTC can be a tortuous process and London Underground has experienced some challenges with such upgrades. The Jubilee line has had a history of problems when, firstly, the original plan for ATO had to be abandoned prior to the Millenium and, secondly, the subsequent deployment of the Thales Seltrac system encountered many problems before commissioning was finally achieved in 2011 in readiness for the London Olympics.
Even the Victoria line upgrade to CBTC using an Invensys (now Siemens) system needed considerable weekend line closures before a phased introduction could take place. However, perseverance is a virtue and the Northern line has recently been fully equipped and commissioned using the Thales Seltrac CBTC technology with only minimum disruption to the train service and hardly any of the users even noticing that an upgrade had taken place. Not being on the front page of the newspapers with reports of travel chaos is testimony to a successful project. Rail Engineer recently visited both London Underground and Thales to see how lessons had been learned.
The Thales Seltrac system
This technology, originally developed for the Vancouver Sky Train network, has been around since the 1990s.
First used in the UK by Docklands Light Railway, the technology was designed for moving block operation using a continuous track-mounted loop that gives full ATO (Automatic Train Operation) and also allows trains to ‘close up’ during high density running.
Each loop is mounted within the ‘four foot’ running rails with a transition every 25 metres where a phase change takes place. Train location is thus confirmed at every transition point and this works in conjunction with train odometry and accelerometers for accurate positioning data. The transmission from loop to train is an inductive couple of around 56KHz.
Movement Authorities (MAs) are generated from the control centre equipment and are calculated in real time. The MA will extend forward as the train in front advances. To maximise capacity, as trains close up and slow down, the MA will allow a following train to get within 50 metres of the preceding train but the precise distance will depend on the gradient at that point and the actual speed of both trains.
Each train receives an individually addressed telegram at least every three seconds or when a new command is given. In the event of no telegram being received in the timeframe, an automatic stop will happen. During normal running, the MA will take the train to the next station stopping point where the accuracy of position is at worst 50cm but is normally ± 10cm to align with platform screen doors.
The core of the train equipment is two VOBCs (Vital On- Board Computers) with associated aerials to read the loop information, one at each end of the train. Each of these has a duplicated computer that requires both to be in agreement before commands to the train traction and braking systems can be transmitted (two out of two configuration). Both VOBCs act independently, with the one adjacent to the driving end normally being used once the driver logs in.
Should this fail, the system will switch to the other VOBC at the rear which then automatically adjusts to the train length when assigning new Movement Authorities. The ‘distance to go’ is always calculated from the front of the train. Connection between the two VOBCs is by both hard wired train lines and a serial data link.
Equipping the Northern line
Both the Jubilee and Northern line upgrades were let as one contract in 2004 with design work taking place up to 2006. From then, a lull in proceedings occurred while Thales and London Underground took stock of how the installation, testing and commissioning would be progressed and to ascertain the demarcation of responsibilities. With the Jubilee line having priority and being commissioned over the period December 2010 to June 2011, extra time was available to plan for the Northern line which was seen as more complex with its two north south routes across central London, complicated junctions at Camden Town and Kennington, and two northern branches.
The architecture of the Seltrac system as applied to the Northern line is structured around a dedicated fibre optic network from the control centre at a new north London site (Highgate) connecting into selected station equipment rooms. From these the information to the loops is fed by co- axial cable. To obtain full resilience, six transmission rings are provided that segment the railway into logical sections.
The trains, known as 1995 stock, had been in service since the latter part of that decade and thus required retro-fitting with the CBTC equipment. This presented quite a challenge as the vehicles had lots of discrete computers for the various train functions coupled with a multitude of hard wiring. Around 2000 wiring changes per cab end, mainly to accommodate the double ended VOBC configuration, had to be carried out in the process of conversion.
This work was done by Alstom, which own the trains and leases them to London Underground, but with the full participation of local staff at both Edgware and Morden depots. Retro-fitting is never easy, each train needing to be out of service for 14 days and costing £400k.
Trains can operate in one of three modes:
» Full ATO mode where the drivers have only to control door opening / closing and press two start buttons to set the train moving. The driver has a screen showing the MA for reference purposes. This is the normal operating condition.
» Protected Manual Mode where the train is manually driven to the limits of the MA with the in-built train protection system causing automatic braking if the MA looks like being exceeded. Drivers may choose to use this mode as a means of greater vigilance when planned trackside work is taking place or when adhesion conditions are poor and more sensitive driving techniques are needed. All drivers are expected to use this mode on occasions so as to remain familiar with the operation.
» Restricted Manual Mode for when the communication link to the train is lost. In such circumstances there is no Movement Authority but trains can be driven ‘on sight’ at a maximum speed of 17kph following a voice instruction from control. Safety protection is maintained by lineside ‘remote secure indicators’ that assure the correct setting of points and also by the continuing function of axle counters to give train position information. This mode is used in the depots to get a train to the system entry point but the occasions when it is used on the main line are now extremely rare.
If a train suffers a major electrical or mechanical failure, a ‘train doctor’ at control talks the driver through various re- set procedures such as power down / power up. If this is not successful, an on-call technician will be despatched to the failed train and, if necessary, arrangements can be made for the following train to push the failure to a stabling siding.
Changeover to CBTC operation
Migration strategy centred around six stages, starting with the short section from High Barnet to West Finchley in February 2013, followed by the section to Highgate including Finchley Central and the Mill Hill East spur.
Having gained some operational experience, the difficult section onwards to Angel and Euston was then tackled including the important Camden Town multi line junction.
Stage four embraced both the central London routes (City and Charing Cross Branches) including the combining line junction at Kennington with its semi-circular reversal loop. Following that, the south line to Morden with its stabling depot was cut over and finally, in June 2014, the section from Belsize Park to Edgware including the depot at Golders Green was implemented.
All this has enabled an increase in capacity with currently 26 trains per hour (tph) on the Bank branch, 22tph via Charing Cross and 30tph down to Morden. The normal service pattern is for Charing Cross line trains to run only as far as Kennington then traverse the reversal loop to run back northwards. Bank line trains run through to Morden. Northbound trains run to Edgware and High Barnet from both the Bank and Charing Cross routes with Mill Hill East being served by a shuttle from Finchley Central.
In peak hours, trains can serve all terminal points from any station and this means a very complex timetable that has the effect of reducing the maximum possible throughput. As the London Underground engineer stated: “If you were building the Northern line again, you would not construct it the way it is.”
At the start of a journey, the driver enters their number on the cab set-up panel and picks up the train timetable number. If these conform to what is expected, the train can set off once the appropriate MA is displayed. Simultaneously, the train goes to green on the controller’s screen whence progress is monitored on the line display.
At terminal stations, or at intermediate locations where the train is due to terminate, the control centre display goes to orange until the driver re-inserts journey information in the far-end cab.
The CBTC system has been linked to the existing platform indication displays giving a more accurate count down to the time and destination of the next arrival. Similarly, the existing track-to-train radio system (Connect) remains in service but with its control consoles transferred to the new control centre.
Learning from past lessons
Both London Underground and Thales were determined not to repeat the organisational and technical problems that occurred on the Jubilee line. A policy of ‘One Team Working’ was instigated with both parties occupying the same premises at Canary Wharf. By doing this, questions that previously took a week to get answered could now be resolved in minutes. Reliability Objective Groups were initiated and a six month period of shadow running was introduced that allowed the system to be soak tested whilst the trains remained driven to the old signalling.
Another important decision was to retain a loop-based system rather than moving to radio transmission in line with general CBTC advancement. This was partly driven by both Jubilee and Northern line contracts having been let at the same time and some equipment having been purchased for both. The need to avoid new untested technology was, however, the overriding reason as this was just too risky. The number of commissioning stages was another factor, limiting these to manageable limits and starting with the simpler was a contrast to the Jubilee line introduction.
The Northern line CBTC project cost has been around £330 million, which included the preparation and independent assessment of a safety case. Also included was the provision of the dedicated fibre network. A study is being carried out to see whether the London Underground Connect transmission network could be used in part as the bearer for future line upgrades, thus offering the prospect of some cost reduction.
An extension of the Northern line from Kennington to Battersea, with an intermediate station at Nine Elms, is now authorised and work will commence shortly with a completion date in 2018. This will connect into the Kennington turn back loop with the planned service being to divert all Charing Cross line trains to Battersea, thus allowing 32tph on both central London branches. Further increases in capacity up to 36tph will only be possible by additional segregation of services at Camden Town, but it is recognised that this will be at the expense of some passenger inconvenience and no final decision has yet been taken.
Thales is also the nominated supplier for the much- troubled upgrade plan to equip the Sub Surface Lines with CBTC systems. Although these too will use the Seltrac design, this time it will be radio-based so a technology change becomes necessary. A preliminary contract is enabling some testing work to be done at the Old Dalby test track where the new S Stock is undergoing commissioning trials. Assuming success, radio technology should set the scene for eventual conversion of the remaining London Underground lines to CBTC operation.