Tunnels on the Great Western main line are poised to take the concept of the Rigid Overhead Conductor-Rail System into uncharted waters, as part of the
£2.8 billion electrification of the railway from London Paddington to Cardiff. If all goes well, this alternative system of delivering power supply to trains could appear in many more UK tunnels.
Electric trains collect 25kV via a pantograph, a sprung, roof-mounted device that exerts a light upward pressure on the conductor wire to maintain contact. Traditionally, copper conductor is suspended from a catenary cable, fixed to successive masts. The conductor, in 1.4km sections, is kept under tension by devices that accommodate expansion and contraction as temperature varies.
Catenary equipment requires considerable headroom. As the pioneer of mechanised railways, The UK built structures to a smaller cross-sectional ‘loading gauge’ than that used overseas. Brunel’s Great Western Railway was built for unusually wide tracks and trains, but the vertical clearance in tunnel and bridge arches was similar to the British norm.
The famous western portal of Brunel’s Box Tunnel, east of Bath, flaunts an arch at least twice the height of trains, but the bore is much smaller. So the tunnel will receive a Rigid Overhead Conductor-Rail System (ROCS) from Swiss manufacturer Furrer+Frey. An overhead aluminium bar will hold the copper conductor in place.
Network Rail will install the same compact system in the 7km-long Severn Tunnel, during a six-week closure from 12 September. The Severn Tunnel, opened in 1886, unexpectedly cut through numerous underground streams, necessitating complex drainage.
A ROCS will also be installed in Chipping Sodbury Tunnel, both bores of Patchway Tunnel and the original bore of Newport Tunnel.
Tested at speed
Since 1984 Furrer+Frey has supplied ROCS for 346 railway locations worldwide, including 181 tunnels. Those tunnels are typically on new high-speed railways – the equipment has been tested by trains exceeding 300km/h in Austria – or on metro systems or older tunnels near stations, where train speeds are relatively low. The UK already has several low-speed installations, including at the Eurostar depot in Stratford, east London.
London’s Crossrail tunnels will receive ROCS, from a different manufacturer.
The Great Western electrification takes Furrer+Frey into new territory. The tunnels requiring ROCS are old but trains pass through at relatively high speeds. In the Severn Tunnel, the limit is 120km/h and the equipment will support future increases up to 160km/h. Those higher speeds demand greater precision where pantograph and conductor meet, but the track lies on a bed of stone ballast. There is a degree of dynamism in such track.
Mr Ankur, Furrer+Frey’s UK engineering manager, says that modern tunnels tend to have rails fixed to concrete beds, ensuring a constant relationship between the position of the track and the ROCS. The Great Western tunnels are different. “Because these tunnels are so old, the track quality suffers and needs careful maintenance,” he says. “So we had to design a dampening system.”
The rigid conductor rail will be suspended from the tunnel roof, rather than fixed directly to it. The dampers will permit some deflection as each pantograph passes. This innovation was trialled on a test track last year.
Last spring, Network Rail installed two 5m test sections of the ROCS in the Severn Tunnel. Some of the parts will be removed in September and examined for evidence of how the tunnel’s environment will dictate maintenance regimes. Gareth Leese, project engineering manager at Network Rail’s Wales Route, says the environment and temperature vary throughout the tunnel, influenced by the large fans that force fresh air into the brick-lined bore. Two of the most extreme environments were chosen for the tests.
Although the tunnel lining has been swept to remove accumulated soot from trains, fresh soot will be deposited after electrification, as only a minority of trains will convert from diesel to electric traction.
Corrosive atmosphere
As for the tunnel’s dampness, Ankur says: “The Severn Tunnel is special because we had to go for a very high grade of stainless steel for components supporting the aluminium bar, because there are high chloride levels in the moisture. A lot of soot has been deposited, which gets mixed with the moisture and can have an impact on the material.”
Track in the Severn Tunnel must be replaced every six or seven years because of this corrosive atmosphere, but the tunnel’s ROCS is designed for an 80-year life. The copper conductor will require replacement periodically but less often than the conductor in catenary systems, thanks to the way the ROCS will grip the wire. The underside of Furrer+Frey’s aluminium bars have a continuous opening for the copper wire. The cable is installed using a device that temporarily prises the sides apart to admit the wire. The sides subsequently hold the wire tightly in place. Ankur likens it to a zipper. The same process will be reversed to remove worn conductor cable.
He explains that the conductor in catenary systems requires relatively frequent replacement in response to wear from the passage of pantographs, because the conductor is under tension. The conductor in a ROCS is not under tension and therefore can experience 50% to 55% erosion before renewal is required.
Any lengthy ROCS requires freedom to expand and contract. On low-speed railways, the start of one conductor-rail section parallels the end of the previous one, creating an overlap. In Great Western tunnels, sections of conductor rail will be separated by expansion joints and consecutive sections of conductor will be aligned laterally. “For high-speed lines, it gives a good performance,” says Ankur.
Where conventional catenary meets a ROCS, a transitional overhead structure is installed, about 6m in length. “It’s a fairly flexible section of conductor rail which gradually stiffens up,” he says. “That avoids any peaks or jerks into the pantograph. It has to be 6m to adapt from the flexible to the rigid environment.”
He argues that the relatively high capital cost of installing a ROCS soon pays off through savings. The reduced maintenance burden is a key factor.
Ankur also points to safety benefits, especially when a momentary gap between the pantograph and conductor results in arcing. “Even a huge spark will only create a minor scratch or small burned-out section on the conductor rail, which will still operate as normal,” he says. “With catenary, you’re talking about a potential dewirement, when the pantograph tears down the conductor.”
Once the Great Western tunnels have demonstrated that a ROCS with dampers is compatible with relatively high train speeds inside sooty, damp old bores, ROCS may be selected for many other British tunnels, predicts Ankur.