Lying abandoned under London in 1984 was the route between Farringdon and Ludgate Hill, known as the ‘Snow Hill Tunnel’, not to be confused with a similarly named line in Birmingham which coincidentally was also reborn in the 1980s. Writes David Bickell

Passenger services ceased at Holborn Viaduct Low Level as long ago as 1916 and the half a mile of track was lifted in 1971 after freight traffic finished in 1969. Reopening the tunnel was considered in a 1974 London Rail Study but it was the erstwhile Greater London Council (GLC) that took the initiative and funded a detailed cost benefit analysis.

The anticipated benefits included opening up many cross-London journeys and better in-town distribution, saving time otherwise needed to use the bus, tube or walk to final destination. From British Rail’s point of view, there was promise of significant operating cost savings. Time required to turn back EMUs at St Pancras, Moorgate, Holborn Viaduct and Blackfriars would be saved by through running.

The Secretary of State for Transport authorised the scheme in 1986. The project involved various construction works including new track, signalling, and provision of 46 new 4-car Class 319 EMUs – the initial timetable envisaged four trains per hour (tph) through the Snow Hill Tunnel.

By now Network South East (NSE) had been created and took the scheme on board, branding the new service ‘Thames Link’ which became operational as a through route in 1988. ‘Thameslink’, as it later became known, quickly became a well established and recognised brand.

Controlling the route

A new double junction was created at Farringdon to facilitate movements to the south via the reopened tunnel, whilst retaining the existing route for some services to continue to Moorgate as hitherto. A new Westinghouse Westpac geographical relay interlocking was installed in Farringdon relay room. This was controlled from the NX Westinghouse panel at West Hampstead Power Signal Box (PSB).

Similarly, near Blackfriars, a junction was created to allow trains to continue to run and terminate at Holborn Viaduct. A new free-wired relay interlocking was provided at Snow Hill to control the line and carriage sidings there. The existing GEC-GS geographical interlocking at Blackfriars was suitably modified. Both these interlockings were controlled from the Victoria ASC (Area Signal Control) South Eastern panel.

In 1990, the Snow Hill route was temporarily suspended to enable Holborn Viaduct station to be closed, the junction removed and the whole site redeveloped. A new box was built for City Thameslink station (the new name for Holborn Viaduct Low Level) on a lower alignment. This had the unfortunate operational nuisance of requiring significant gradients at either end of the station.

Capacity increase

Thameslink settled down, rapidly becoming a huge success. However, the capacity limitations began to show as commuters experienced severe overcrowding at peak times. Network South East developed plans to increase capacity, dubbed the Thameslink2000 project with the intention that the scheme would be implemented by the turn of the century.

Alas, the project met with numerous delays on the journey including rail privatisation, two public enquiries, and the Olympics, leading to the inevitable jibes of ‘Thameslink 3000’! In December 2006, Network Rail was granted the planning permission and legal powers it required to execute the project. The scheme was finally given the green light when funding was confirmed in July 2007.

Although the scheme had been fully developed by Network South East, technology had moved on and ridership continued to grow. Accordingly, some elements of the original scheme were revised. To meet the continuing growth, a high capacity signalling scheme was required to deliver 24tph through the core area between Kentish Town and Blackfriars, rising potentially to 30tph for an hour to support service recovery. Some novel features were included in the signalling to ensure that performance risk would be minimised in the core section. Any delays here would rapidly spread and affect the Brighton main line, East Coast main line, Midland main line and beyond.

Key Output 1

From the inception of Thameslink, trains had a mixture of 4-car and 8-car. The new requirement for 12-car operation necessitated extension of the platforms at Farringdon. Due to the 1-in-29 gradient approaching from the north, the only option was to extend the platforms at the Snow Hill end by building out across the formation of the line to Moorgate. Whilst the loss of the latter service would inconvenience some commuters, the removal of the double junction here would substantially reduce signalling fault potential and contribute greatly to the robust and efficient operation of the core.

At Blackfriars, a major exercise was undertaken to remove the conflict between Thameslink services running to/from London Bridge with services from the Loughborough Junction direction terminating in the bays located on the eastern side. The bays were relocated to the west side and the Thameslink double track slewed to the east side.

By a dint of good forward planning, when St Pancras station was being transformed to become ‘International’, provision was made for a concrete box to be built underneath that would accommodate the Thameslink lines and a possible new station. Whilst initially not funded, the station box was subsequently fitted out and opened for business enabling the cramped old Kings Cross Thameslink station to be closed.

These developments have required significant signalling alterations.

At Farringdon, the relay room had to be demolished to facilitate the extension of the platform. The new layout at Blackfriars bore little resemblance to that previously signalled. Thus the whole core route was completely resignalled in good time before the start of the Olympics of 2012 and, of course, is designed to support the eventual 24+ tph. The new signalling allowed the introduction of 12-car trains on the route for the first time and First Capital Connect has immediately taken advantage of this with the provision of additional rolling stock to ease overcrowding. Today there are 15tph.

High capacity, high availability signalling

Mid platform signal with POSA [online]The contractor for these schemes was Invensys Rail (now Siemens Rail Automation). Tried and tested signalling kit has been installed. New Westlock computer interlockings (Solid State Iinterlocking derivatives), controlling lineside Trackside Function Modules (TFMs), were installed at the respective control centres at Victoria and West Hampstead. Signaller interface is achieved via Westcad work stations.

Train detection utilises Bombardier EBI Track 200 (TI21) frequency track circuits which provide immunity to AC and DC traction systems. There is a block section every 70 metres with associated electrical rail terminations.

Major new projects on the mainline generally choose axle counters as the preferred method of train detection. However axle counters are unsuitable for the core given the long trains, short block sections and mid-platform signals. This combination could result in wheels stopping over the transducer heads giving rise to axle counting errors and hence the train detection section failing ‘occupied’. Furthermore, once an axle counter section has failed, for whatever reason, it cannot be restored to the ‘clear’ state without a time consuming reset process.

Modelling demonstrated that 3-aspect signalling would suffice for normal scheduled operations but this didn’t provide the flexibility to recover from perturbation. Hence a 4-aspect sequence is used throughout the core and mid-platform ‘headway’ signals allow a following train to proceed into a platform as soon as the rear of the previous train has cleared it rather than having to wait for the ‘starting signal’ overlap to clear.

Signals have been positioned such that a full 12-car train can stand between stations, with the platforms at either end also occupied. A miniature LED signal head has been developed for use in cases of restricted clearance such as in a tunnel. AWS (Automatic Warning System) and TPWS (Train Protection & Warning System) is fitted as standard.

Limited bi-directional signalling is provided in conjunction with strategically placed crossovers by which trains may be turned back. Although full bi-directional signalling was specified for the original NSE scheme, it is now considered that any attempt to implement signalled single line working, say around a failed train, would severely compromise capacity and bring the service all but to a standstill. Other measures will be in place to get a failed train out of the way quickly.

Eagle-eyed travellers will have noticed what appear to be position-light ‘call-on’ or ‘shunt’ signals associated with all the main running signals.

In fact, they are part of the strategy to keep things moving whenever possible.

Degraded mode operation

Traditional signalling is designed to be ‘fail-safe’. This means that any failure within the signalling system should cause associated signals to display the most restrictive aspect. There is no technical back-up. The train service comes to a standstill whilst anachronistic, time-consuming manual procedures are implemented, usually involving verbal communications between driver and signaller/handsignaller. On a high speed main line or complex suburban area, extensive delays quickly spread, giving rise to a large number of delayed passengers. Of course, it was always the case that engineers would be given personal objectives to reduce signalling failures. The reality is that, even in the 21st century, the harsh railway environment in which signalling components perform their task is such that a fault free utopia is some way off.

POSA signals

When Railtrack took over administration of the infrastructure, business managers were extremely concerned at the financial impact of failures under the Schedule 8 performance regime designed to compensate train companies for poor performance attributable to the company. A work stream, encouraged to think ‘out of the box’, started to look at the feasibility of running trains under degraded conditions using signal aspects. A significant output from the process was the concept of the ‘Proceed on Sight’ aspect. After much discussion this concept was ratified by the publication of Railway Group Standard GE/RT8071 in 2006. A ‘POSA’ consists of two white flashing lights set at 45°. A signaller may set a POSA route if a signal fault is present such as track circuit failure, as long as point detection is confirmed and in all other respects the interlocking can permit the aspect. The driver may proceed and be prepared to stop short of any obstruction.

Retrofitting this feature requires costly modification of existing interlockings, particular if they are hard wired rather than electronic. However, inclusion of the facility at the design stage of a new project is much easier to do and should pay dividends in years to come by limiting the performance implications of track circuit failures. POSAs have been fitted to all running signals in the core.

Emergency point and route releases

In addition to holding signals at red, track circuit failures also deadlock points thereby preventing the signaller/ARS (Automatic Route Setting system) from changing the route. To overcome this further impediment to the running of trains under failure conditions, another novel feature has been introduced. Emergency point and route releases allow the signaller to change the route without the involvement of the signal technician.

Key Output 2

Returning briefly to St Pancras International, at the same time that the station box was being constructed over ten years ago, two tunnels were bored between the East Coast main line at Belle Isle and just north of the St Pancras box. These tunnels pass under the Regents Canal and have subsequently been dubbed ‘Canal Tunnels’. They plug Thameslink into the East Coast main line, allowing Great Northern services to become part of the Thameslink portfolio serving Cambridge and Peterborough. Phased installation of the physical junctions and signalling work to bring these tunnels into service has commenced this autumn. The tunnels were described more fully in issue 89 of The Rail Engineer (March 2012).

At Blackfriars the service splits with a planned 18tph going forward via London Bridge, the remaining 6tph continuing via Elephant & Castle.

Currently, from Blackfriars onwards towards East Croydon, Thameslink services face severe bottlenecks. Between Blackfriars and Metropolitan Junction there is even a section of single line, followed by a short double track section towards London Bridge shared with 29tph serving Charing Cross. Platforms are shared at London Bridge with Charing Cross services and, at the country end, there are flat junction conflicts. Because of these capacity constraints, during peak periods most Thameslink trains are diverted via Elephant & Castle and unhelpfully don’t serve London Bridge.

Hence the 24tph Thameslink service remains an aspiration until the challenge of the capacity constraints in the vicinity of the ‘Bridge’ can be fixed. The good news is that the project team are currently preparing for Thameslink to have a dedicated double track route all the way from Blackfriars through London Bridge to Bermondsey. Thameslink will have separate platforms, the trains linking into the East Croydon route near Bermondsey where a new-grade separated junction will obviate conflicting moves. There will be various stage works through to scheme completion in 2018, with signalling controlled by workstations eventually to be located at the Three Bridges ROC (Railway Operating Centre). In 2012 Invensys Rail was awarded the contract by Network Rail for the London Bridge station area resignalling. This will include overlaying ETCS (European Train Control System) and ATO (Automatic Train Operation).

A variant of ETCS

With the Cambrian Early Deployment Scheme already implemented, ERTMS (European Rail Traffic Management System) progress in the UK and evolving concepts have been described in The Rail Engineer before. However, the Thameslink scheme has some unique features.

A frequent 30mph metro style operation is required through the London core but, out on the Midland main line (MML) and East Coast main line (ECML), 100mph capability will enable the trains to share the route with inter-city services. South of London, Thameslink services will run at a variety of speeds, intermingling with Gatwick and Brighton expresses, and suburban services in Surrey, Sussex and Kent.

Given that Network Rail is planning a national programme of ETCS fitment, the logical decision was taken to use ETCS level 2 as the platform for ATO. However, conventional lineside signals will remain to provide a ‘degraded mode’ option. Movement authorities will thus be issued in parallel to the lineside signals via conventional datalinks, and to ETCS system for display on the Driver Machine Interface (DMI) in the cab.

ETCS communications between the control centre and the train are via GSM-R and the Radio Block Centre (RBC). An analysis of system capacity has considered the number of train movements and anticipated volume of voice and data calls. The final design of cell size and base station infrastructure will ensure there is sufficient capacity and there will be a single RBC to manage all movement authorities through the core area and London Bridge station.

There will be 580 balises mounted in the four-foot to transmit positional information to the onboard train systems. In the core area, ETCS End of Movement Authority (EOA) positions will coincide with signal positions with the ATO driving to a stopping position some nine metres in rear of the signal.

A novel feature at key locations to improve throughput and assist with perturbation will be the creation of shorter ‘virtual block’ sections within the normal signalled block sections. In this case the movement authority will read up to a ‘yellow on blue’ block marker rather than an illuminated signal. Block markers are a familiar scene on the Cambrian lines and HS1. These additional sections will apply only in ETCS mode to allow a following train to move further ahead.

ETCS is due to go live in 2015. There are no plans to migrate to ETCS Level 3. Understandably, the project team needs to gain in-service experience and understand the implications of leaving signals in situ before moving to the next level.

First application

ATO is already in use on four London Underground lines and the DLR but this is the first application on Network Rail infrastructure. On arriving at the ATO boundary the driver will see a flashing yellow button. To accept automatic working the driver will put the power controller to neutral and press the yellow button. This will normally happen on the move.

Simulations of the core train service using the VISION model which supports driving profiles and dwell times have demonstrated that ATO together with 45 second dwell times is necessary to achieve the desired capacity. ATO receives timetable and regulation commands from the ATSS (Automatic Train Supervision System) at the control centre, transmitted via Packet 44 – a facility used to transmit separate data using ETCS communications and STMs (Special Transmission Module). ATO is a non-vital system that requires a safety backup to prevent a SPAD (Signal Passed At Danger) or over-speeding. That takes the form of ATP (Automatic Train Protection), part of the ETCS functionality.

Following the Ladbroke Grove serious SPAD in 1999, train companies developed ‘defensive’ driving standards which understandably tend to err on the side of caution when approaching yellow and red signals.

In a high-capacity metro-style system trains need to be driven in an identical manner, optimising the use of the line speed profile and braking curves, accurately stopping at platforms and red signals (plus or minus half a metre) without braking-coasting-braking. In fact, the ATO system will decelerate the train at rates that result in the ETCS speedometer displaying warning of potential intervention but avoiding actual intervention.

ATO will also open the doors immediately the train stops, and generally help to minimise station dwell and dispatch times. 45s dwell time is specified, 30s for passengers to embark/disembark and 15s for train dispatch. The driver will be presented with a cab count-down display in seconds to ensure prompt departure at the specified time. Dcanal tunnels track [online]oor closing will be a manual process by the driver, ensuring passenger safety.

In the core, trains will stop with the centre of the train lined up with the centre of the platform. This, and the elimination of 4-car working, will obviate the current performance issue where a short train stops in an unexpected position leading to a sudden mass movement of passengers along the platform just at the moment those already on the train are attempting to find space into which to alight. It will also assist disabled passengers to position themselves in a suitable boarding position. Incidentally, platform doors are not being fitted at core stations due to the curvature and vertical geometry of some platforms.

In the event of a fault with the onboard ATO, the system will degrade to manual driving with ETCS. Further levels of degraded operation are to lineside signals with AWS/TPWS, and finally driving on sight with POSA signals.

Overall supervision

The ATO will work in conjunction with ATSS, a traffic management system that regulates train movements in accordance with the timetable, has built in strategies to deal with perturbation (ie re-plan the timetable), and issues route setting requests to ARS. The ATSS will transmit regulating instructions to the onboard systems, appropriate for the conditions ahead. An energy-efficient speed profile may be selected but, for example, if a train is running late then ATO may drive as fast as possible.

Outside of the core area, a DAS (Driver Advisory System) will provide the energy efficient driving advice to the driver (see issue 104 – June 2013).

The traffic management system at the ROCs will be crucial to getting an even-interval service through the core by ensuring that trains present themselves at St Pancras International and Blackfriars on time and in the right order, and that trains can leave the core as efficiently as possible.

New trains

To meet the capacity requirements, a brand new fleet of Siemens Desiro City Class 700 trains is under construction. With a top speed of 100mph, these will be introduced from 2015, being driven initially in manual mode. There will be fifty-five 12-car and sixty 8-car trains, each car being only 20 metres long because of the tight clearances on the route. These will be fitted out with ETCS functionality version 2.3.0d and ATO. AWS and TPWS will be fitted for operation outside the core and for manual driving mode within the core.

The trains are dual voltage with traction changeover at Farringdon (southbound) and City Thameslink (northbound). If a northbound train fails the changeover from DC to AC it can be immediately driven out of the way into the turnback sidings. In ATO mode, traction changeover will be accomplished automatically within the 45s dwell time.

The trains are currently being manufactured in Germany and the first to be rolled out will be tested at the Wildenrath test centre next year.

Operating instructions, Rule Book and staff training

Running a railway with ATO and conventional signalling involves a complex set of operating procedures. Building on the work already undertaken on the Cambrian ERTMS scheme, significant additions and amendments are needed with the national Rule Book to accommodate ETCS and automatic operation. The project team are working with FCC and the Rail Safety and Standards Board (RSSB) to bring this to fruition by 2015.

ATO operation will introduce a new perspective for train drivers. For example, they may find the faster approaches to red signals a little scary. Training will ensure they gain confidence in the new systems before taking a live train through the core in ATO mode.

System proving and integration

The new ETCS and ATO train control technology must undergo a rigorous process of development, testing and commissioning. This cannot be achieved on a working railway. Furthermore, other suppliers are developing kit for Network Rail’s national roll-out of ETCS which also requires testing. Accordingly, a new facility called ENIF (ETCS National Integration Facility) has been set up using part of the Hertford loop using a specially adapted Class 313 unit. Fortuitously, the signalling, ETCS, ATO and new trains for Thameslink are all now being supplied by Siemens, thereby keeping contractual interfaces to a minimum.


The project team are extremely busy ensuring the many challenges of equipping this route for high capacity train movements are realised. Come 2018, long suffering commuters will have a world class mainline/ metro service and experience at least a 200% increase in capacity at peak times.

The survival of the Snow Hill route connecting the networks north and south of the Thames was probably a close-run thing. It was the extraordinary vision and determination of the transport planners of the GLC and BR’s NSE who rescued the line from almost certain destruction by redevelopment in the 1980s.

The primary source for this article is a recent presentation to the IRO by Paul Bates, project director, Thameslink Programme, and an earlier paper to the IRSE by Paul Bates and Dave Weedon, Thameslink principal signal engineer.