Signalling Metrolink

Taking a brief step back in time, on the morning of 17 July 1992, with camera in hand, I jostled for a good viewing position with the crowds gathering in St Peter’s Square. It was a very special day for Manchester. Her Majesty the Queen was officially opening the new-fangled Manchester LRV (Light Rail Vehicle) system. After unveiling the plaque, the Queen travelled in tram number 1010 to a civic reception in Bury.

In those days the term ‘tram’, seen as old fashioned and inappropriate for the gleaming new LRVs, was suppressed from publicity but the incomprehensible acronym LRV (light rail vehicle) never really registered in the mind of the public. In the twenty-first century, taking the tram is cool and the distinctive toot widely recognised as they ply the streets of Manchester.

How it started

Metrolink was born out of a long standing frustration by local transport planners of the historical north-south divide of the heavy rail routes serving Victoria station on the north side of the town and Piccadilly to the south. In the 1970s, the so called Picc-Vic scheme envisaged the two mainline stations being connected together for through running by means of a tunnel under the city centre.

Alas the project was deemed unaffordable but, not to be outdone, the planners came up with the ingenious idea of using LRVs to link together the Altrincham and Bury suburban routes by means of a street-running section through the city centre, with a spur from Piccadilly Gardens to Piccadilly station. The 1992 line was 19 miles long, served 26 stops and was operated by 26 Firema T68 trams (all now retired). There were 60 drivers and 20 line controllers. This Phase 1 was followed by Phase 2 to Eccles in 2000. The first twenty years of progress was fully described in issue 86 (December 2011).

25 years later

Fast forward 25 years to 2017 and we reach completion of the current Phase 3 extensions with the opening of the Second City Crossing (2CC), the 14.5km Airport line having opened in November 2014. The sheer scale of the works in progress since 2008 can be gauged from the fact that the network will be three times the size of the original, extending over 60 miles, serving 93 stops and operated by a fleet of 120 new Bombardier Flexity Swift M5000 trams with approximately 300 drivers and 60 line controllers.

Between now and 2017 the 2CC is under construction together with additional platforms at Victoria, Deansgate, and St Peter’s Square. The latter location will see two new island platforms with four tracks replacing the existing two. In a similar operation to the current Victoria stop closure, single line working using a Tram Token will be introduced for ten months.

And the story continues. High up at lonely Pomona, passengers aboard a Salford Quays bound tram notice a short stub as if the line was intended to continue straight ahead whilst the tram swings sharp right to cross the Manchester Ship Canal. Indeed, this is the junction for the long-anticipated 5.5km extension to Trafford Park for which public consultation took place this summer.

The organisations involved today

Transport for Greater Manchester (TfGM) owns Metrolink and is the body responsible for implementing local transport policy. Metrolink RATP Dev UK Ltd (MRDL) operates the overall network and maintains the Phases 1 and 2 infrastructure until 2017.

The M-Pact Thales (MPT) consortium was awarded contracts for the Phase 3 extensions commencing in 2008. MPT carries out all the design, construction and maintenance of the new lines. Laing O’Rourke and VolkerRail (jointly known as M-Pact) delivers all the civil and rail infrastructure requirements respectively, and Thales UK provides all the electrical systems engineering works, including power and overhead line equipment, and also the tram management system (TMS) for all lines under a standalone contract. The MPT contract is also due to expire in 2017.

Parsons Brinckerhoff is TfGMs delivery partner, providing a comprehensive management service including programme and project management, risk management, project controls and contract management for the Phase 3 expansion programme.

Signalling the trams – Phase 1

Phase 1 employs solid state interlockings (SSIs) for all rail junctions and relays to control the plain line signal sequences. On the segregated routes, track circuit block is in force, using two-aspect red/green aspect signals (with some yellow/green repeaters where stop signal visibility is sub-standard) supported by the automatic tram stop (ATS) which is a signal passed at stop (SPAS) mitigation measure consisting of electromagnetic beacons that apply the brakes of a tram that passes a red signal, similar to TPWS on the main line. Maximum line speed is 50mph.

On the street-running sections in the city centre, trams are driven on sight at a maximum speed of 30mph. Colour light signals cannot be used here due to the potential for confusion with road traffic signals. Hence the continental-style white bar type tram signals are deployed with a horizontal bar indicating stop, and a vertical bar meaning proceed on sight.

There are no block sections in the city centre, trams being driven such that they will stop short of any obstruction be it a bus, pedestrian or another tram. Points are called by a message transmitted by the tram vehicle recognition system (VRS) to inductive loops in the track. In the control centre at Queens Road, the controllers set routes and monitor the progress of trams on the VDU screens much like their main line counterparts.

Drive on sight

With the planned rapid expansion of the system, it was felt that a block signalling system was not going to be able to support the desired headway between trams (zero minutes!) and the level of flexibility required in the new complex network of routes. TfGM took the bold decision that the entire network would be run on the ‘drive on sight’ philosophy allowing trams to closely follow one another. The new system has been implemented on the lines built since 2008 and in the city centre, with gradual roll-out proceeding outwards towards Bury and Altrincham concurrently with recovery of the redundant block signalling and associated signals.

TfGM is keen to stress the flexibility and throughput of trams facilitated by the new system. This is well illustrated standing on the platform at Cornbrook where trams arrive and depart in rapid succession which was not possible with the older, block system. It also gives a positive perception to passengers of a slick healthy system. During special events or match days, trams can queue up at stations and help move the masses on their way home very quickly.

After all, drive-on-sight was already the modus operandi for the city-centre section from day one and, on the faster segregated sections, is driving a tram really any different to driving a bus?

It’s not quite that simple. TfGM is responsible for setting line speeds and has carried out a system-wide exercise to ensure that speed limits are consistent with available sighting time, driver reaction time and the ability of vehicles to stop. The line speed on segregated sections is 50mph which may be lower in places as determined by the line-of-sight principle. A particular sighting problem occurs within Collyhurst and Heaton tunnels where sight lines are compromised by line gradient and curvature. This scenario is covered by an innovative solution provided by Thales. Axle counter sections will detect the presence of a tram in the tunnel and switch a variable speed sign to display a lower speed limit for a following tram.

Drivers are instructed not to drive faster than they can see to be clear ahead so as to stop with a normal service brake. A driving simulator for MRDL has been developed by Ian Rowe Associates, enabling drivers to be trained in a modelled virtual environment before taking trams along the actual routes.

Route setting

Unlike the Phase 1 scheme and main line signalling, there is no centralised route setting and interlocking. The next journey, derived from the timetable, is downloaded to the trams in the form of a ‘trip’. All the driver has to do is press the ‘ready to start’ (RTS) button and away it goes, the tram automatically routing itself at junctions. As all trams have the same driving characteristics and stop at all stations along the intended route, there are no regulating decisions to be made requiring complicated algorithms.

At the 25 key rail junctions where routes diverge and join, routes are set on a first-come-first-served basis. However, in the event of an incident that could well have a knock-on effect on the whole network, controllers may need to step in and make changes to the timetable and/or individual tram schedules to minimise delays and aid service recovery for which TMS provides the tools. Controllers can edit the timetable and hence trips for a particular tram, the period, or for the rest of the day. They can also switch to a mode of operation known as ‘headway operation’ where trams are separated on a headway rather than timetable basis.

As a last resort, the driver is able to override the timetable trip and to turn back short and set up an alternative destination. Several intermediate turnback locations are provided on the network. The driver enters a routing code on his key pad to call the points required for the crossover move. Line controllers can temporarily block a route if there is a need to hold a tram or give priority to another tram, but this is unusual.

For route setting, trams automatically transmit a route call message via a transponder on the underside of the tram to the local controller by means of ‘advance’ and ‘stopline’ inductive detector loops in the four foot. When the junction becomes available, the tram gets a proceed indication. Signals are normally at stop and change to proceed as the driver approaches. At facing points, a route indication is provided by means of a separate points indicator which is an illuminated orange sign confirming detection of ‘points left’ or ‘points right’.

Multi-level control

Junction control consists of three tiers. Firstly, on the network there are roughly 100 local controllers – off the shelf single processor industrial programmable logic controllers (PLC) – which receive the call from a tram and follow a fixed sequence of logic conditions to determine if the route is available. If so, the points controller (SIL3) drives the points and ensures points can’t move under the tram. On street areas, this is achieved by means of a short track blocking circuit detecting the body of a tram. Off-street sections are locked for a greater distance using a Thales axle-counter product.

Thirdly, the conflict monitor (comparable to SIL2) independently checks the state of the signal and points, checks the integrity of the equipment and notes when trams pass the SPAS detector beyond each signal. It is capable of putting all signals back to stop if it detects a malfunction or a SPAS scenario and, in the case of the latter, will also illuminate flashing blue lights (off street areas only). It is a matter for tram drivers to react accordingly. A ‘clear’ loop detector at the exit of the junction frees up the points for the next tram.

In the case of the 100 or so road intersections, the local controller interfaces the road traffic light controller logic. Trams enjoy a level of priority at all road crossings, as determined by TfGM, which has to ensure overall transport flows efficiently including buses for which it is also responsible.

Where a tram stop is located just prior to a junction or intersection, the driver presses the ‘ready to start’ button when ready. This avoids a premature call for a junction or crossing that may delay other trams or road traffic.

If communication is lost with the central control, trams continue to run as points and signals are set locally.

Knowing where the trams are

The position of a tram in the network is detected by the local controller using physical detection points supplemented with position reports from the tram via the Mesh radio. The vehicle identity is forwarded from the local controller via the fibre network to the network management centre (NMC). Vehicles use the loops to calibrate their location on the track.

In between the physical loops there are incremental position reporting locations known as ‘virtual detection points’ based on a pre-set distance from the last physical loop. The tram onboard computer (OBCU) has a database of topology and knows the positions of the virtual loops. When the OBCU measures that the tram has reached a virtual detection point using information from the vehicle odometer, a location update radio message is transmitted over the nearest Mesh radio access point thence via the fibre network to the local controller.

On the road sections, the majority of position reporting points are virtual loops as this obviates the difficulty of faulting and maintaining physical loops in the highway. Thus, on the highway, signals at road intersections are called almost exclusively by radio messages.

Physical loops are provided at every rail junction, terminus, turnback location, siding and every exit from depot – a total of 400 loops. In addition, there are 1,100 virtual loops making a total of 1,500 detection points altogether. At NMC, tram identities are displayed alongside loop identities on the line controller screens looking similar to a mainline train describer display except that loop identities replace signal numbers.

Integrated supervisory system

Line controllers at the NMC are responsible for the complete portfolio associated with managing the service. In addition to tracking the trams, the supervisory system supplied by Thales fully integrates with overall tram management, the passenger information system (CIS), and the SCADA front end which controls traction power. It also provides comprehensive status reporting from external equipment including security and ticket vending machines and auxiliary equipment such as lifts and escalators.

Passenger information is derived from the timetable which is generated by the operator MRDL’s own timetable system which in turn is linked to the driver rostering system. From the timetable, a vehicle duty is extracted which is automatically updated to the tram OBCU. Early predictions for CIS are taken from the timetable and, as the tram progresses along the network, displays both on the CIS and on the vehicles are updated to the real-time situation.

There are automated announcements on PA using pre-recorded speech with facilities to put out manual announcements. Announcements in Greek and Spanish are known to have been played during European fixtures. The introduction of TMS has thus improved the whole customer experience.

Communications

The entire tram network now uses an extensive IP- protocol Gigabit fibre-optic Ethernet LAN with highly resilient diverse routing, carrying data for TMS, SCADA, equipment monitoring, CIS, CCTV, telephones, help points, emergency call points and some of the voice radio communications from base stations. Migrating the existing Siemens 36Mbit OTN fibre network has been a challenge with the need to run the two networks together for a time. Rigorous out and back testing went well although one or two cameras could not be converted due to them using an old protocol.

Drivers communicate with NMC via the existing industry-standard MPT-1327 analogue UHF trunk radio network which has been expanded by Thales to cover the new lines. It is kept separate from the data communications.

The technical challenges

Thales recently elucidated to The Rail Engineer the sheer scale and complexity of the TMS project, not to mention the many overall Phase 3 project engineering interfaces. Many sections of route are commissioned, in service and supported, whilst other sections are still in design and manufacture of equipment.

Management of a single team in software and hardware working at the different stages of project life cycle and locations is a huge engineering challenge in itself, even without the changes in requirements as the work has progressed. Thales have some 250 people working on the project with a factory at Cheadle for design, build and test. A significant consideration has been the TfGM requirement that a custom solution should use off-the-shelf components from multiple suppliers.

All the Phase 3 lines are operated using TMS with the shared parts of the original routes being the first to be migrated, leaving the outer sections of the lines to Altrincham and Bury still to be converted. A batch of fifty-two M5000 trams are ‘dual-fitted’, having ATS functionality enabling them to work over the original routes as well the new lines. There are temporary transition points for tram drivers as TMS is progressively rolled out towards Altrincham and Bury respectively.

Separate consoles are used in the control centre for TMS and legacy signalling, the track diagrams being updated at each stage of changeover to TMS and special interfaces provided for tram position indications. Care has had to be taken at design and during decommissioning to avoid the risk of interaction and electromagnetic interference between the two systems, track circuits, inductive loops, ATS electromagnetic beacons. Both systems use physical loops on track for street running with transponders on trams.

As transponders cannot be mounted on top of each other, the legacy system transponders are fitted under the centre bogie of the trams whereas the TMS transponders are at each end with a 15m separation. The transponder loop at the leading end is active.

Testing of the new system loops is carried out overnight with a procedure that involves switching off the old equipment, activating the new, carrying out the tests, then reversing the process to revert to the legacy system until a new section of line is wholly tested and ready to be permanently changed over to TMS. Work is going on at multiple sites.

Civil and track engineering for the Phase 3 lines was running slightly ahead of the complex development of the overall TMS package. With commercial desirability to open new routes for business at the earliest opportunity, Stockport- based company Park Signalling Limited was contracted to provide temporary signalling interlockings to control Trafford Bar, Irk Valley Junction (Smedley Viaduct) and the single line at Dean Lane. Although the interlocking function was implemented using conventional relays, the PSL design was novel in that treadles were used for tram detection and the existing vehicle recognition system (VRS) was used to call
the routes from the tram. Treadle and conflict management was implemented using Siemens S7 PLCs.

Last but by no means least, another big challenge was the relocation of the NMC from Queens Road depot to larger premises at the new Trafford base.

The overall project design and construction has been undertaken in accordance with the ORR’s publication ‘Guidance of Tramways’ whilst the safety verification has been steered by ORR’s ‘A Guide to Safety Verification for Tramways’ to achieve the requirements in the ‘Railways and Other Guided Transport Systems (Safety) Regulations 2006’ (ROGS).

A very successful 25 years

Metrolink has become established as a very successful transportation system, taking up the course of some old mainline railway routes and breaking new ground in areas not previously served by rail, providing comprehensive journey opportunities. Ridership is heading towards 30 million passenger journeys a year, up 50% in five years and 20% on last year. There is no doubt the Metrolink Silver Jubilee anniversary in 2017 will be a cause for celebration, paying tribute to the far sighted transport planners who conceived the LRV system back in 1980s.

Many thanks to Joel Sawyer, communications manager TfGM; Daniel Vaughan, head of operations TfGM; Stephen Corlett, project design authority TMS, Thales UK; John Gerrity, engineering manager, Thales UK; and Andy Jenkins, systems engineering manager, Thales UK, for their help in the preparation of this article.