London Overground’s North London line (NLL) runs from Richmond in the west, south of the Thames, over Kew bridge and then in a broad arc around the north of London, through Acton, Camden and Hackey out to Stratford in the east.

The line is fully electrified, and uses third-rail DC from Richmond to Acton Central and then overhead 25kV AC over the rest of the route. Four-car Class 378 Electrostar trains, built by Bombardier, service the route with a minimum of four trains an hour throughout. There are also four London Overground trains an hour on the connecting West London line which gives a total of eight trains an hour on the Willesden to Stratford section.

Drive to increase capacity

In common with most railways, demand on London Overground is rising and capacity is constrained. The solution, as it was on the network’s East London line (ELL), was to lengthen the trains from four to five cars. However, it wasn’t that simple as thirty-one platforms across 17 stations were too short for the new, longer trains. So Transport for London (TfL) established the London Overground Capacity Improvement Programme (LOCIP) to deliver the infrastructure, stations and rolling stock works to enable five car trains to operate across the network.

The works on the North London line are funded by Transport for London under an asset protection agreement with Network Rail. Once complete, the asset reverts to Network Rail which will undertake routine maintenance. This is a good example of London Overground investing in Network Rail’s infrastructure to the benefit of other rail users as well as TfL. On this route, a lot of freight crosses London using the NLL.

The ELL part of the programme is now substantially complete. New storage sidings at Silwood, a modified depot at New Cross Gate, platform extensions to take five-car units and signalling modifications are all now in service and the longer trains are being introduced.

Work is also progressing on the North London line. London Overground has taken over the ‘C’ sidings at the Wembley Freight Operating Centre. This five-road facility is having two sidings, numbers two and four, removed to leave space between the three remaining roads for concrete walkways and watering and cleaning facilities for the newly-extended passenger trains.

The Willesden Traction Maintenance Depot (TMD) is being partly extended and its sidings reconfigured for the longer trains. New diesel sidings are being built to continue to accommodate the diesel trains that operate the Gospel Oak to Barking line.

Longer stations

So that leaves the 31 platform extensions. Most of these are fairly straightforward and principal contractor Dyer and Butler is approaching these in a conventional manner. Some need a little piling to provide a good foundation. Then it is a question of cross-walls and precast concrete slabs or of blockwork walls and beams. Copings and tactile strips go on top with a concrete screed and/or macadam surfacing to finish it all off. Nothing terribly exciting, but good bread-and-butter railway engineering.

In line with the Government’s target of delivering all major projects with Level 2 BIM, the designs were carried out using 3D modelling within the Projectwise collaboration tool.

The works, like most railway schemes, are logistically challenging with the route crossing six London Boroughs and many of the stations being constructed in fully urban environments. This has resulted in many platform extension areas backing onto residential properties and gardens and access being extremely difficult. Stakeholder interface and management of public relations during the logistical planning of the works has taken a high profile on this scheme. Both the methodology and design of the product has been hugely impacted by the community with a big focus on ensuring all stakeholders are fully engaged with the works being undertaken.

gospel oak CREDIT TONY GEE & PARTNERS [online]

The programme is demanding with a construction period of less than 12 months to design, construct and commission the 31 platform extensions. Construction mainly takes place during weekend possessions to ensure minimum disruption to the day-to-day operational railway.

Some were tricky

While most of the platform extensions were reasonably straightforward, with the complexities being as much logistical as technical, a few required a bit more engineering.

At Acton Central – one of the first platforms lengthened – the platform had to be extended over an area of poor ground with numerous immovable and ‘irregular’ high-voltage cable runs. Dyer & Butler, assisted by designers Tony Gee & Partners, developed a scheme combining small-diameter grundomat piling with an adjustable steel platform substructure, giving maximum positional flexibility for placing piles within the constricted area.

To extend the platforms at Brondesbury, Dyer & Butler and Tony Gee developed a scheme combining temporary and permanent works to stabilise a historically-unstable embankment and enable heavy piling equipment to work alongside the track.

A number of “unknown critical structures” – four large Victorian brick arched voids – were encountered under one of the platforms at Camden Road giving the team two problems to solve – what to do with the voids and how to build the extension over the top of them. Utilising laser-cloud survey techniques, Dyer & Butler established the size and condition of the brickwork structure forming the voids without needing to gain physical entry into the potentially-hazardous confined space (with a long drop). This enabled collation of site survey data of a sufficient quality to enable Tony Gee to develop a simplistic solution.

Gospel Oak probably had the most ‘engineering’ to it. The eastbound platform was long enough for the new trains already. However, the westbound one was not. Already extended in 2010 when the London Overground trains were lengthened from three cars to four, the proximity of Gospel Oak Junction (the connection with the Gospel Oak to Barking line) meant that further extension at the western end of the platform was impossible.

The only solution was to go the other way, over Gordon House Road and the ticket office which was already in the process of being upgraded.

Dyer & Butler and Tony Gee came up with a solution that eliminated loading on the old station ticket hall by developing a ‘drop-in’ steel support for the main bridge section over the road. This transfer loads from high level down to a piled foundation in the ground through a ‘keyhole’ opening in the ticket hall roof. Low-headroom piling rigs were used and steps were taken to limit the inconvenience to users of the booking hall.

More piling took place on the other side of the road and a new two-span composite structure was installed. The spans were lifted in over a weekend using a road crane parked on the closed Gordon House Road with the 10-tonne main span being successfully placed onto the new steel supports which had been threaded into the existing booking hall.

Interestingly, the designs for the prefabricated spans incorporated permanent steel formwork on which the in-situ concrete deck slab could be quickly cast.

“Dyer and Butler did a great job,” commented London Overground’s head of programme delivery Hugh Lawson afterwards. “We only had one opportunity to lift the bridge in. Everything went according to plan and they handed the road back precisely on time. It was nicely done.”

The two platforms at Kentish Town West are on a brick viaduct. The arches underneath are occupied, most of them by Camden Brewery. “They have lots of fork lift truck movements,” Hugh explained. “So we had to make sure that they weren’t inconvenienced too much.”

A simple steel frame solution was proposed by Dyer & Butler’s site team which was developed by Tony Gee and manufactured and installed by Mcnealy Brown. This was erected on both sides of the viaduct on piled foundations, and concrete deck slabs laid on top. Once again, copers, tactiles and a screed layer went on top of that.

Thereafter, the new platforms were finished off as all of the others. Lighting on the existing stations was reasonably new, much of it dating back to the 2010 extension programme, so it was mostly a question of patching into that. Likewise, drainage, CCTV and passenger information were integrated with existing services.

Still more to go

Work is well underway at all of the stations. At one stage during October Dyer and Butler was working on fourteen sites at once, and everything is looking good for the introduction of the new five-car service before the end of 2015.

Mike Stubbs, TfL’s director of London Overground, commented: “The platform extension work and 25 per cent capacity increase the train lengthening programme will bring, is part of a wider expansion plan for London Overground which has become one of the most popular and punctual railways anywhere in the country.

Kentish Town West_Piling for platform extension (2) [online]

“It has brought previously under-used parts of the urban network back into full use, while services have been radically improved with continuing investment and expansion to parts of the capital once not well served by rail. Stations have been brought up to modern standards, with CCTV, better security, a turn up and go service for disabled Londoners and staff present at every station at all times.

“From 31 May, TfL will look after three quarters of all journeys in and out of Liverpool Street station – having integrated West Anglia and Liverpool Street to Shenfield rail services into the TfL network. The West Anglia services will join the London Overground Network, and the Liverpool Street to Shenfield services will be run by TfL as the first phase of the introduction of TfL-run Crossrail. As has been witnessed in other parts of London, integrating these services will bring new opportunities, support future growth and unlock regeneration.”

What’s happening with Gospel Oak to Barking?

The Gospel Oak to Barking line, currently operated by diesel-powered Class 172/0 trains, has been the subject of several electrification proposals over recent years. In the main, these came to nothing due to a lack of funding. However, it has been confirmed that the line is now to be electrified. Part-funded by TfL and with backing from the Department for Transport, the scheme is being delivered by Network Rail. Currently at Grip 3 (option selection), completion is provisionally set for the end of 2016.

TfL is implementing a scheme which involves extending the platforms on the route for four-car electric multiple-unit trains. In addition, an order will be placed for trains to replace the aging West Anglian fleet, which comes into London Overground from Abellio Greater Anglia on 31 May 2015, with options for additional trains for capacity enhancement and to replace the diesel fleet on the Gospel Oak to Barking route.

Delivery of the new electric sets is not likely to be before 2018, leaving a period where the existing diesels will still operate under the new wires. However, freight operators will benefit from the electrification of this line as soon as it comes into service.