London Underground’s Jubilee Line has had an interesting history.
The Stanmore section was originally part of the Metropolitan Line but became a branch of the Bakerloo Line in 1939 when the deep level tube was opened from Baker Street to Finchley Road to relieve congestion on the Met.
Subsequent overcrowding on the Bakerloo central section led to a new SE – NW tube being built in 1979 across central London from Baker Street to Charing Cross and this was joined to the Stanmore branch to create the Jubilee Line.
The original plan was to extend this along the route of the Strand into London’s east end but the route was changed to include Waterloo and London Bridge, thus giving these places a direct link to Stratford and the growing Docklands conurbation. This meant abandoning the Charing Cross terminus with the extension diverging to Westminster after Green Park station.
The Jubilee Line Extension (JLE) opened in time for the Millennium. The line is 36.2km long, has 63 trains, 2 depots and carries between 7-800,000 passengers per day.
Controlling the Trains
It had been intended to open the JLE with Automatic Train Operation (ATO), but delays and technical difficulties meant a somewhat rushed conventional signalling system with train stops had to be installed so that the line could be operational for the Millennium celebrations.
The underground parts of JLE were equipped with platform screen doors and the drivers became very skilled at stopping the trains accurately so that both train and platform doors coincided. Manual driving was only seen as a temporary situation since the desired number of trains per hour could not be achieved by this method.
Accordingly, a new contract was let with Thales to provide their Seltrac system, as pioneered on Vancouver Skytrain and now used by other metro systems including Docklands Light Railway. Andrew Hunter, the Chief Engineer for the Seltrac system in the UK, gave a detailed explanation on how it works.
Termed as Transmission Based Train Control (TBTC), the basis of the Seltrac system is continuous communication to the trains by means of inductive track loops laid between the rails.
A loop has a maximum length of one kilometre and is transposed (crossover) every 25m to give position reference points. Each 25m section is sub-divided into four 6.25m segments, thus giving 160 positions in the one kilometre section.
The train transmit antenna has a frequency of 56KHz and the receive is 36KHz. Seltrac is a moving block system controlling the safe separation of trains and the safe movement of points.
The target point for a train movement authority is the distance from the rear of the preceding train. A distance of 50 metres between trains can be achieved.
Seltrac is designed around six main component parts:
The System Management Centre (SMC). This is the operations room located at Neasden and housed in a modern, high security building. From here, the line is controlled by eight work stations arranged in a circular architecture. Additional work stations are provided for timetable compilation and maintenance, this latter sited in the equipment room. The operator interface system is regarded as non-vital in safety terminology. A separate training room is equipped with simulated line conditions where staff can be confronted with typical problems and failures.
The Vehicle Control Centres (VCC). In old speak, these are the interlockings but, in addition, they provide all the commands to the trains plus point setting. Five are required for the Jubilee line to cover the full extent of the line and are the vital safety equipment rated at SIL4. Inside each VCC are three types of rack: data transmission, input/output rack and the processor rack that uses the traditional ‘2 out of 3’ principle for assured safety.
The Inductive Loops, described above, which are connected directly to the VCC.
Station Controller Subsystems (SCS). These communicate with the VCC and adjacent SCSs to control the status of emergency stop devices, point position, axle counters and other peripheral equipment within their area of operation.
Platform Door Interface Units. These communicate to both the VOBC (see below) and VCC via a docking loop to open and close the platform screen doors when a train is at a station.
Axle counters, provided for when the ATO system has a problem and the trains have to be driven manually. These allow separation of the line into discrete blocks such that reversion to fixed block working will occur when operating in degraded mode. In ATO conditions, more than one train can be in these block sections.
In addition to these six main components, there are also rail gap indicators consisting of 3 red lights that indicate a ‘dead’ section of track ahead; train ready to start indicators consisting of 3 white lights operated by platform staff at terminating stations; route secure indicators at points for use in degraded mode to show the path the train must follow; direction indicators giving a text display used for when traversing non passenger moves into and out of depots.
All infrastructure facilities are linked to the SMC by an independent fibre-optic cable network giving a main and standby path to each device.
On board the trains, two main components are provided:
Vehicle On Board Controller (VOBC). This vital system gives the Automatic Train Protection (ATP) and ATO interface to the traction motors and brakes, and is responsible for train movement in line with the distance and speed commands communicated from the VCC.
Train Operator Display. Fitted in the cab, it shows the movement authority that has been given together with target and actual speed. In ATO mode, the driver merely observes events but in degraded mode, he/she must drive the train as closely as possible to the permitted movement.
Seltrac in Operation
The TBTC system has four modes:
- Automatic – driver responsibility is only for door opening and closing plus pushing the start button;
- Protected Manual – driver drives to the speed profile displayed as communicated from the VCC;
- Restricted Manual – driver drives on ‘line of sight’ to a maximum permitted speed of 17 km/h;
- Off – no train movements permitted with emergency brake applied and propulsion system disabled.
Normal operation is ATO with the others being increasing fall back (or degraded) situations. As a train enters a loop for which a movement authority is granted, the VOBC counts its position in that loop from an onboard tachometer and reports back to the VCC.
As the train progresses so its position is monitored against the allowed movement. On approaching the end of the authority, the train will be slowed to stop at the correct position in a platform or at a safe distance from the preceding train.
An accelerometer detects whether wheel slip or slide is occurring and adjusts the speed profile accordingly. Engineering trains cannot operate in automatic mode.
Jubilee Line Depots
To house and service the trains, the line has two depots; the main one at Stratford Market plus a stabling one at Neasden. Operating these depots efficiently and safely is itself a major logistics exercise and each requires its own signalling system.
Stratford Market movements are controlled from the depot control tower, which commands a good view of the entrance tracks, the maintenance shed and the stabling sidings.
Conventional shunt signals and points, controlled from VDU screens by mouse, give drivers authority to progress to the exit point. Here the changeover to ATO is effected, this being co-ordinated with the SMC at Neasden.
Normally, trains exit to Stratford station for the start of the in-service journey but an alternative exit towards West Ham is available involving a reversing move inside the depot.
So important is the depot signalling system that a standby hard-wired control panel is provided should the main screen based system fail. A further VDU screen enables control of the traction current to the conductor rails.
Train maintenance and routine servicing is undertaken in the multi-track shed where power for the trains is provided from an overhead gantry so as to avoid the safety hazard of conductor rails inside the depot.
Also at Stratford is an indoor training facility for the Seltrac signalling system complete with test track, inductive loops, VCC and other peripheral equipment.
At Neasden, the Jubilee Line sidings are adjacent to the main Metropolitan Line depot and the combined line usage has demanded a more sophisticated control system.
Thales have provided the equipment using their Locktrac PMI architecture. The 4 MEI interlockings are based on industrial PCs configured as programmable logic controllers (PLC).
Safety is assured by two ‘2 out of 2’ computer systems and is SIL4 rated. Auto changeover happens on a regular timing to ensure that all PLCs are functioning correctly.
Three layers of software are used – generic product; generic application; specific application. The interface to the main line signalling of both the Metropolitan and Jubilee lines is via the control rooms at Baker Street and Neasden respectively.
Duplicated banks of four screens provide the operator interface while axle counters in the depot sidings provide the train detection. Distribution of commands to the signals and points is via a TCP/IP Ethernet fibre network.
Phase 1 of this system, covering most of the depot, was commissioned in October 2011, with Phase 2 being planned for November 2012. A question as to whether this technology could be used in a main line application, was answered by; well, why not?
The Jubilee Line in Service and the Future
Londoners who travel regularly will know from press reports that many problems were encountered during the service introduction of the Seltrac system. Andy Bourne, the Upgrades Manager for Tube Lines, explained the contractual structure: London Underground is the system operator, Tube Lines were responsible for the delivery and integration of the system including all enabling works and ongoing maintenance, Thales were the supplier of the signalling system.
Since Seltrac was a mature system with many years of reliable service on other networks worldwide, it could have been expected that its introduction to the Jubilee Line would be trouble free.
However, the complications of having to retrofit the trains and superimpose the new infrastructure on to a working railway would always have been a challenge, but adding in the various mix of technologies including platform doors and the line interfaces to Metropolitan and Bakerloo lines made it all a difficult task.
Developing the remote secure routing for trains in Restricted Manual mode of operation was a new requirement and this took time to get right.
After trials on a dedicated test track at Highgate, ATO was introduced on a gradual basis, the final section being the Stanmore branch in mid 2011.
Reliability was initially very variable with non-communicating trains being the main problem in early days. Even now, hardware failures still occur too frequently but it is getting much better.
A further series of hardware and software improvements will be implemented before the Olympics. From July 2011, a service of 27 trains per hour was introduced, rising to 29 from March 2012.
The use of diagnostic tools has proved invaluable since TBTC generates huge volumes of data and interpreting this is, in itself, a difficult exercise. Distinguishing between infrastructure and train borne faults is not necessarily easy.
The big test will come with the Olympics, when it expected that daily ridership will rise to an expected two million per day necessitating 31 trains per hour.
An identical Seltrac system is currently being installed on the Northern Line and with, all the lessons learned, it is anticipated that this will have a smoother introduction. T
he Seltrac technology will probably be updated to a radio based solution, thus not requiring track loops, but whether it would ever be worthwhile converting the Jubilee line is dependent on the business case and is as yet an unanswered question.
For now, the transformation is complete and passengers are benefitting from the upgraded service.