A new fleet of heavy-duty, national rail engineering trains was successfully put into action on the London Underground network over the weekend of 19/20 August 2006 by Metronet Rail, the company responsible for the maintenance and upgrade of two-thirds of the Tube.

Metronet has already renewed more than 55 km of track since it began work in April 2003 and has a further 19 km of track renewals planned for this year – 10 km more than the previous year.

The new fleet brings the best available technology and methods from the national rail network to support this massive programme, delivering a step change in Metronet's renewal capabilities and delivering to London Underground and its passengers journeys that are more reliable, smoother and faster.

Ultimately the fleet will double the length of track typically renewed in any one weekend from around 500 metres to 1 km, as well as triple the number of worksites Metronet can operate on the sub-surface lines of the Metropolitan, District, Hammersmith & City and Circle lines.

The weekend's work was treated as a trial with Metronet renewing 476 metres of near life-expired track at two sites on the southbound track at Amersham on the Metropolitan line.

The locomotives, on their first outing, were used to pull trains almost 400 metres in length, with wagons and the latest in high-output track laying equipment. This was the first time in the history of the network that Class-66 diesel locomotives had worked on London Underground infrastructure having started their journeys from locations on the national rail network.

The fleet is owned and operated by GB Railfreight under a 10-year, £80 million service contract with Metronet. Metronet Chief Executive Andrew Lezala said: “For the first time, Metronet has brought heavy-duty diesel locomotives and high-output track-laying equipment from the mainline railway to work on the open sections of the Underground. These 126-tonne locomotives are capable of pulling trains and machinery such as Slingers and auto-ballasters up to nearly half a kilometre in length which allows us to deliver new track quicker than ever before. This innovation has been two years in the making and brings the best available technology and methods to get the job done as quickly and as efficiently as possible meaning a better journey for the three million people who use the Underground every day.”

Metronet has had to overcome many challenges in bringing the new fleet to the Underground, including satisfying London Underground's stringent vehicle approval process. Class 66 diesels are much heavier and larger, principally in height, than existing locomotives so bridges are being strengthened on the District line to pass Metronet's own strict structural assessment criteria.

The Class 66 locomotives are manufactured by EMD in Ontario, Canada. Being capable of pulling formations up to 430 metres in length cuts the number of trains needed to service each weekend work site from a maximum of 30 trains to just eight, introducing new efficiencies.

The five new Class 66s form part of an £80 million investment by Metronet Rail, under a 10-year contract with GB Railfreight. The deal will provide Metronet with an engineering train fleet of nine Class 66 locos, 2 further diesel Schoma locomotives, 153 wagons, a Slinger train, and three tamping machines.

This new fleet will increase Metronet's track replacement capacity on the sub-surface lines of the London Underground (the Metropolitan, District, Circle, Hammersmith & City and East London lines) from 15 weekend work sites per year to a minimum of 50.

The new fleet will work on the open sections of the sub-surface network, on those sections of the Metropolitan, District and Piccadilly lines that can be accessed from Network Rail infrastructure. The new fleet will be stabled off the Underground network, including at a dedicated depot at Wellingborough, Northamptonshire. The engineering trains will arrive during possessions only and not run in operational traffic. They will be brought on to the Underground network at existing boundary points at Amersham, Harrow-on-the-Hill, Gunnersbury, East Putney, Wimbledon and a new link proposed at Upminster.

Work with the new fleet began this weekend with renewal of a stretch of track between Amersham and Moor Park on the Metropolitan line. Later, work will progress to Preston Road on the Metropolitan line and additionally cover the District line: east of Bromley-by-Bow, south of West Brompton down to Wimbledon and west of West Kensington to Ealing Broadway (this includes sections of the Piccadilly line).

GBRf is investing £15m to provide: 9 Class 66 locomotives, 25 side tippers, 90 Falcons (wagons), 10 auto hoppers/ballasters, 8 Tench and 14 Salmon wagons. In addition, Trans Plant (a part of Tube Lines) will be providing through GBRf 1 new Slinger train, 2 plain line tampers and 1 points and crossings tamper and crew. Additionally, Trans Plant will provide a train each weekend to enable early delivery of material and tools. The latter will normally comprise 2 Schoma diesel locos and 6 general purpose wagons.

Class 66 locomotives have a maximum speed of 75 mph and slow speed control of 0-10mph to two decimal places. When operating on London Underground/Metronet infrastructure the GBRf trains will always be in possession and limited to a max speed of 15mph – this is because the Class 66 locos will not be fitted with LU train-borne safety devices (tripcocks) and because of the increased length of trains (up to 430m) and the interface with LU infrastructure which is designed to accommodate a maximum train length of 137m.