Corsham in Wiltshire is a deceptive town. On the face of it, it has a picturesque main street with aged stone buildings. Gentile shops and olde worlde pubs. It’s a picture postcard town – or so it seems. Sure, there’s the never-ending roar of the A4. After all, this was the old London Road before the M4 was built.

So where’s the mystery? The mystery is underground. Deep in the Oolite limestone in a network of tunnels that would have been – and who knows, might still be – a regional seat of government in the event of nuclear Armageddon. Corsham isn’t as innocent as it appears.

Brunel drove his new Great Western Railway through – or rather just to the south of – Corsham, hacking through the Box Hill – his famous Box tunnel. And it’s from the eastern portal end that there is, or rather was, an access to this other subterranean and secret world.

Electrification is coming

This is just setting the scene. There’s a much more prosaic world of Corsham, of course, especially around the normal Pound Mead housing estate – built just to the south of the underground labyrinth and near to the site of the old Corsham station. This closed in 1965 but there’s still a useful piece of land for railway engineering works. Off Pound Mead runs a footpath to a neighbouring estate, across the railway and over a footbridge.

As all must be aware by now, electrification is coming to the Great Western main line. With it comes the need to ensure correct electrical clearances and, guess what, the old footbridge failed the test. Electrification has been on the cards for so long now that this was not a surprise, but when the green light was given it was time to sort out a definitive scheme. This was worked up from basic requirements by Atkins until a final design involving a standard London Midland steel footbridge was adopted.

The neighbours

Once upon a time, this footpath and bridge was in open countryside, but now it’s smack in the middle of housing with very constrained access. It was obvious early-on that the neighbours needed to be kept informed of all the works and especially what was going to happen when the old bridge came out and the new one was lifted in. The proximity of all these constraints drove the design. Raising a footbridge doesn’t just affect the main span structure. There are ramifications for the approaches as well. Inevitably they need to be longer – but in the case of the bridge at Corsham, the lengthenings were kept to a minimum by hogging the main span to achieve the lift of approximately 800mm.

Despite its lowly origins, the footbridge was well used and so the diversion of 1km during a complete closure had to be sensitively negotiated. A temporary structure was erected which carried the inevitable diverted services – water, electricity, telecomm cables and the like. Diversion works took place from 10 December.

Up and over

The new bridge was fabricated by Nusteel and transported from Kent to the old station yard ready for the 27-hour possession on 21/22 March. Main contractor Hochtief hired a 500 tonne crane from Ainscough and lifted the new span in over the houses. The existing footbridge bank seats were retained so avoiding the need to construct new footings.

The occupants had been given the opportunity to spend the night in a hotel while all this went on. Both the Network Rail and Rail Engineer web sites have an impressive timelapse video of the whole operation.

Relations with the neighbours have remained positive throughout and all that remains now is for the approaches to be completed and the services to be diverted back onto the new bridge. As Minhaz Uddin, Network Rail’s project engineer says, “On the next day we were greeted by residents very pleased with their nice new footbridge – with its ‘holly green’ paintwork!”

Another secret

Normality will return to Corsham although there is the possibility of the old station reopening. And the hidden subterranean world just a few hundred yards away wasn’t affected in the slightest. It just sits there waiting for our next national crisis.

But maybe the best-kept secret in Corsham is ‘Cinnamon’, a really excellent restaurant in the High Street. Well worth a visit.