Gospel Oak to Barking renaissance

Although listed in the 1963 Beeching report as “All passenger services to be withdrawn”, the fourteen mile Gospel Oak to Barking (GOB) route, hitherto something of an urban backwater, managed to survive. Today, like the North London line to which it is connected at the western end, it is experiencing a dramatic resurgence.

To double passenger capacity, a £133 million project (GOBE) to electrify the line is being managed by Network Rail. The sections being electrified include the core route from Gospel Oak to Woodgrange Park Jn, Harringay Park Jn to the East Coast main Line and the short run into Barking bay platform. Short stretches at South Tottenham and Woodgrange Park to Barking are already wired.

Traffic development

The role of the route has evolved significantly in the last half century. Once, it played host to a variety of suburban services including North Woolwich to Palace Gates and St Pancras to Barking, not forgetting the ‘bucket and spade’ specials heading to Southend-on-Sea and boat trains between St Pancras and Tilbury, all now long withdrawn.

By 1980, the passenger service had reached the nadir of an unreliable hourly service between Kentish Town and Barking operated with ageing DMUs. In 1981 this was replaced with a new hourly Gospel Oak to Barking service which has gradually improved over the years with newer rolling stock and today deploys modern Class 172 two-car Bombardier Turbostar DMUs running at four trains per hour. With passenger ridership doubling since 2008, there is an urgent need to increase capacity, and the project will facilitate the introduction of four-car EMUs to double capacity.

The route also plays host to intermodal freight traffic from Tilbury and the London Gateway deep-water port towards the West Coast main line, and this is also showing strong growth. However, GOB is a non-electrified island with connectivity to many other key routes that are energised at 25kV AC. This limits the usefulness of the line.

Electrically hauled freight from the aforementioned sites in east London have to travel from Barking via the flat junction at Forest Gate, a manoeuvre difficult for signallers to execute without causing delays to trains on the Great Eastern main line. Eliminating these moves will improve infrastructure capacity and performance on the GEML and the Elizabeth line (Crossrail).

The strategic value of this route, facilitated by electrification, was recognised in the Network Rail Electrification Route Utilisation Study (RUS) of October 2009. The scheme showed a Benefit Cost Ratio (BCR) of 2.4, representing high value for money. Funding was not included in the High Level Output Specification for Control Period 5 and the high price tag for electrification, the reason for which will become apparent in a moment, led to some misgivings, but was eventually forthcoming with a contribution of £108 million from Department for Transport (DfT) and £25 million from Transport for London (TfL).

Collaboration

Network Rail is the self-appointed principal contractor, which ensures that access can be given to contractors according to rail business priorities. In order to undertake this role, Network Rail has brought in additional project management resources from Collaborative Project Management Services (CPMS), which specialises in the delivery of complex, multidisciplinary projects where a number of different contractors are required to work in close collaboration.

J Murphy and Sons is the main contractor, undertaking track and civil engineering, including bridges and provision of viaduct brackets for the OHL masts, and the minor S&T works. Stobart Rail is carrying out track work with Rhomberg Sersa brought in for its slab track expertise.

A joint venture between Amey and Inabensa is providing the works for traction power and Overhead Line Equipment (OLE) design. Network Rail’s in-house Overhead Condition Renewals (OCR) team is providing the OLE installation. This new national resource was in-sourced from the West Coast main line delivery project in 2006 to address the growing electrification workload in CP5 and provides a full package from design to installation including planning project resources and possessions, and fabrication of components. Last, but not least, the Aspin Group is installing the OHL mast foundations.

Keeping ahead of conflicts and pitfalls

Freeform has provided an advanced 4D Modelling tool using in-built Synchro software. Visualising the construction process is easier to follow and enables faster project decision-making than Gantt charts would otherwise do. The model includes engineering train arrangements and all plant and machinery.

Rhino software creates 3D elements whilst Grasshopper runs dashboards to display key data and conflict analysis, generating a virtual construction sequence. Network Rail reckons the cost of the 4D model is paid for by identifying and avoiding conflicts in advance.

A general problem is that Victorian infrastructure is not always accurately recorded on plans. Detailed site investigations were carried out to try and identify the location of services in order to minimise the risk of any nasty surprises during construction, although some unforeseen issues do still arise.

Track and civil engineering

The project is ostensibly one of ‘electrification’, but installing and powering the Overhead Line Equipment (OLE) actually comprises only about 22 per cent of the work. So what is absorbing such a significant chunk of the time and budget?

The line, built by two separate companies between Gospel Oak and Tottenham North in the 1860s and from South Tottenham in the 1890s, presented costly engineering challenges. There are at least forty structures of various configurations that pass over the railway, typically road, rail, utility services and footpath overbridges. These are concentrated in four major sections where cuttings were created by cutting a slot through the London clay.

The concept of electric trains powered by a pantograph drawing power from an overhead conductor didn’t come about until a trial in Baltimore in 1895, so future-proofing the line for electrification wasn’t a consideration of Victorian railway builders. Hence the legacy for today’s project is many structures with insufficient clearance for the OLE, insufficient clearance for rail vehicles at the haunches of the arch, insufficient space to pass overhead line catenary and insufficient parapet heights.

Consideration was given to bridge jacking or reconstruction. This has an impact on services and utilities such as telecommunications, electricity, water, gas and sewers – which are numerous and buried in the bridge. The sheer number of diversions that would be required, and the extensive collaboration between the various authorities necessary to meet project timescales, means that this is the non-preferred option.

However, four bridges are being rebuilt including the A1 Holloway Road overbridge which is being raised at Christmas as a separate project by TfL. Parapet walls are being raised to a height of 1.8 metres at over 22 bridges to improve safety.

The preferred solution of track lowering is taking place at four significant sites, the biggest at Walthamstow Queens Road involves 16 overline structures in quick succession, including an unusual lattice retaining wall support structure and a bridge with inverted arches. Typically, track is lowered by 200-250mm with a worst case of 500-550mm.

To achieve longevity of the ballasted track to 40-50 years, and maximise the maintenance intervals to 10-15 years, a 100mm layer of sand is over-laid with a geotextile layer to prevent clay pumping. Ballasted track has the disadvantage that it is a little flexible, giving rise to a vertical movement as trains pass over which may compromise the required electrical clearances. Also, when tamped, ballasted track may get lifted, thereby compromising clearances.

One particular challenge is Pretoria Avenue Bridge 57, which contains a Victorian main sewer leaving only 200mm available to lower. This is only just sufficient, but the solution here is the provision of slab track which, by virtue of its rigidity, holds the track absolutely firmly in position, reducing the need for maintenance tolerances.

Slab track solution

The slab track solution is deployed in three sections under bridges and in the tunnel at Crouch Hill, totalling approximately 1,400 track metres. Slab track is quick and easy to install, the pre-fabrication process reduces man- hours on site and requires less future maintenance, lasting for 60 years before maintenance intervention is necessary.

The project team would like to have installed slab track on the entire route but, at a cost considerably higher than that of ballasted track, a business case is not justified on this commuter railway, whereas on a high revenue route like high speed rail, slab track achieves a big return on investment. However, on the authorised extension at the eastern end of the route to Barking Riverside, slab track will be used throughout as the capital is available.

The Rhomberg Sersa Rail Group is the slab track specialist providing the STA (Slab Track Austria) product which has recently been installed on the EGIP Project
at Winchburgh and Queen Steet. The principal element of this system is the elastically supported slab blocks of 5.6 metres length. The slab is an untensioned reinforced precast slab with integrated rail support seats. Holes with a threaded bolt enable fine lift and cant adjustments to the track. A set of steel shims can be put on one side to allow cant transitions from one horizontal curve to another.

The bottom of the slab, as well as the tapered openings, are attached with an elastomeric layer. The result is double-layered elasticity, attenuating vibrations or structural-borne noise, and decoupling from its structural supports.

A joint width of 40 mm separates two slabs and compensates any deformations caused by creeping, shrinking or temperature changes. The joints serve also as surface water drainage or spaces for cable-crossing.

The slabs are supported and fixed on a 250mm base layer of reinforced concrete onto which is put 40-80mm of grout beneath a rubberised layer. This grout is poured through holes to key in the slab section to the concrete below. Should a defect arise, it is possible to break out the pocket, re-insert the key, take out a 5.6 metre section and replace.

Rhomberg Sersa will also be installing twelve V-TRAS transition units, one at each interface between the slab and ballast. These units deal with the normally problematic areas where the change in track stiffness can cause long term performance issues.

OLE structures

The line is to be electrified with the Network Rail Series 2 catenary system, which has capacity for speeds up to 100mph and is designed with fewer component parts than Series 1 which is fit for 140mph. In addition, it has greater maintenance tolerances and is less obtrusive, which offers the added benefit of creating fewer signal sighting conflicts.

During ‘Safety by Design’ workshops, the structures were designed to be sited further away from the track at 2.5 metres, rather than the typical 1.6 metres from running edge, to aid signal sighting and to obviate the need for moving cable routes and created unobstructed walking routes. This adds cost but has large safety benefits. Single span OLE structures are being used where possible to reduce impact on neighbours.

Network Rail’s OCR team has planned the OLE and is delivering masts, booms and wires. Amey and Inabensa are providing all traction power, taken in from the west at Gospel Oak and east at Barking with a new section switching station in the middle at South Tottenham.

This allows the route to be split electrically into two: Gospel Oak – South Tottenham – Woodgrange Park. Foundation installation has been undertaken by Aspin, working during weekend possessions ahead of the main blockades. 550 piles will be needed for OLE support structures.

S&T work

Electrical interference from 25kV AC electricity may cause S&T equipment to fail, so appropriate measures of immunisation must be applied. Parts of the route already pass near to 25kV AC lines and will already be AC immune. However, on the long plain line sections of GOB, earthing has to be installed at every equipment location case and every signal. In addition, a new return screen conductor will be installed to remove stray currents from both railway infrastructure and lineside neighbours.

Six miles of new cable troughing route are required, whilst older asbestos troughs are removed. Where the track lowering groundworks impacts the cable route, cables have been lifted out and temporarily suspended in protective black plastic tubes. One signal is being repositioned and one new banner repeater provided where sighting of the associated stop signal has been compromised.

Currently, the line is controlled from four signal boxes at Upminster, Upper Holloway, South Tottenham Station Jn and Liverpool Street. The project is not realising any additional train path capacity but a resignalling scheme is under consideration for the next control period (CP6) to increase throughput and line speed. For reasons of signalling braking distances, line speeds will also remain the same as at present, varying between 20 – 55 mph although the track works will facilitate higher speeds when the resignalling is implemented.

Accommodating new trains

Where track lowering has taken place, as at Walthamstow Queens Road, platform heights have also been correspondingly adjusted. With the recent upgrade of some sections of the Overground from four to five car operation, the project team is ensuring that the sponsor’s remit to operate four-car EMUs will be future proofed against any future operation of five-car units, though a return to 12-carriage trains, as in days gone by, will be for another generation of engineers to manage.

Although operating at the same four trains per hour frequency as now, the new Bombardier Aventra Class 710 four-car EMUs currently on order for the London Overground will double passenger capacity on GOB and should start to enter service in December 2017.

Closures and possessions

The project has been split into two major blockades to facilitate 24-hour working within which the track lowering work is proceeding. The most complex lowering is at Queens Road, hence this section of route had the benefit of this summer’s blockade. Preliminary access prior to the blockades, mainly undertaken at weekends, has enabled foundations for OLE masts, 95 per cent of the pile foundations and the track lowering at Hornsey Road to be completed.

Timelines for the project are as follows:

• October 2015 – Preparatory work started;
• 4 June 2016 to 23 September 2016 – No service between South Tottenham and Barking on weekdays and no service between Gospel Oak and Barking on weekends;
• 24 September 2016 to early February 2017 – No service on the entire line between Gospel Oak and Barking;
• Early February 2017 – Line reopens;
• March to June 2016 – Weekend closures for testing and commissioning;
• Early 2018 – New Class 710 electric four-car trains enter operation.