Passenger trains are finally running from London Paddington to Cardiff using electric traction the whole way, following the electrification of the Severn tunnel.
As Rail Engineer reported in February 2020, electric services commenced on 7 January but had to continue running under diesel power through the Severn tunnel. This was because the overhead electrification systems was suffering from rapid corrosion in the tunnel which meant that an electrical supply couldn’t be sustained.
Whereas the overhead line equipment (OLE) in open country is a copper wire, kept under tension and suspended from a catenary arrangement, this is not the case in the tunnel. Instead, an extruded aluminium beam is rigidly fastened to the tunnel roof and insulated from it. The copper contact wire slots into that beam and, as it is supported for its full length, doesn’t need to be under tension.
However, under testing, the aluminium beams were corroding much faster than was expected. There was some speculation that an anaerobic bacteria was to blame, as has been experienced on oil rigs, which was ‘consuming’ the aluminium beams’ protective oxide layer and allowing corrosion to proceed.
Noel Dolphin, from OLE designer and supplier Furrer+Frey, explained that two actions were taken to solve the problem.
Firstly, the contact wire was changed to an aluminium one. That made the overhead system ‘all aluminium’ so there was no possibility of galvanic corrosion between copper and aluminium.
Secondly, some parts of the tunnel – perhaps 20 per cent was Noel’s comment – were very wet from water leaking through the lining. The Severn tunnel has always been wet – during construction in 1879, the workings were flooded by water, not from the River Severn, but from the Great Spring which flowed into the tunnel from the Welsh side. Large pumps were installed to control it and these have to run continuously – over 14 million gallons of water are pumped out of the tunnel every day by Sudbrook pumping station.
Readers may be interested to know that some of the water is pumped to a brewery in Magor, near Newport, where it is turned in Stella Artois and Boddingtons!
But the water causing the problems for the electrification wasn’t all fresh water – some of it was saline from the Severn estuary and covering the OLE system with salty water (a good electrolyte) wasn’t a good idea. So Network Rail worked hard to replace drip pans and grout the tunnel lining, all to control the water and make sure that, if leaks couldn’t be prevented, at least they ended up in the drainage system and not on the OLE.
These two measures have resulted in a much drier tunnel and an electrification system that doesn’t corrode. Job done!
The OLE was energised over Easter, since when test trains, and the occasional in-service train, have run through the tunnel under electric power. Those tests were successful, so passenger services can now run through all-electric for the first time.
Sudbrook pumping station is also set to be renewed as part of the upgrade of the Severn Tunnel, further improving the reliability of the railway.
Mark Langman, Network Rail’s managing director for Wales and Western, said: “I am absolutely delighted that the Severn Tunnel is now fully electrified, resulting in a seamless rail link for passengers between Cardiff and London Paddington.
“Electrification has reduced journey times between South Wales and London by as much as 15 minutes and provided an additional 15,000 weekday seats compared with a year ago, with the possibility of further increasing the number of services and seats from south Wales in the future.
“It has been a hugely complex task to electrify the tunnel but I’m thrilled that the final piece of the puzzle is now complete.
“I would like to thank passengers and lineside neighbours for their patience over the past decade as we worked to deliver the transformation of this vital railway and am pleased that they will benefit from these improvements for years to come.”