An innovative system for the electrification of railway track is helping engineers overcome space constraints in the 130-year-old Severn Tunnel, in the UK.

Instead of traditional overhead wires, the system being installed in the 7km-long tunnel (and several others along the Great Western route between London Paddington and Cardiff) consists of an aluminum rail, held to the tunnel roof by drop tubes and registration arms, with the contact wire that carries the power supply fed into a slot in the base of the rail. Installation is being carried out by the Alstom, Babcock and Costain (ABC) Electrification joint venture.

The overhead conductor-rail system is more compact than a traditional wired system and thus more suitable for the space-constrained tunnel. Image credit: Costain.While the Rigid Overhead Conductor-rail System (ROCS)—manufactured by Swiss company Furrer+Frey—has been employed elsewhere in Europe, this constitutes its first use in the UK on ballasted tracks bearing speeds of up to 125 miles per hour.

“It’s a more robust system than overhead wires, with reduced maintenance,” says Costain Project Manager John Skentelbery. "It also gives improved contact with the pantograph—the device on the locomotive’s roof that conducts the power supply down to the locomotive’s motor."

In some tunnels, track levels have to be lowered in order to accommodate electrification equipment. ROCS is significantly more compact than the traditional wired system and can be used in tunnels where headroom is constrained.

“Remember that these are Victorian tunnels that were never designed for electrification," says Skentelbery. "We’re trying to minimize the amount of intervention needed on the track.”

Beyond space constraints, ABC Electrification has had to cope with an array of other factors in their retrofitting of the track infrastructure through the tunnel, including drippage of saline water from the Severn Estuary above it and soot deposition from freight trains carrying coal that pass through it. Those factors mandated the use of high-grade stainless steel with a lifespan of 60 years for the metalwork holding the ROCS to the tunnel structure.