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New route to Letchworth

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At the very moment the 13:57 Moorgate to Letchworth train pulls out of Hitchin making its way to the Cambridge branch, a south bound express thunders over the ECML junction. Up until Monday 26 June this year, this is something that just could not have happened.

The local train would have had to wait or the express would have been stopped. But now the long awaited Hitchin flyover is open for passenger traffic. As if to emphasise the point, a north bound HST adds to the cacophony of trains passing through the station. The Letchworth train serenely accelerates northwards, unperturbed by the movements on the two fast lines following the ‘feather’ of the newly commissioned starting signal.

An announcement is made by the driver to reassure those regular passengers expecting to turn right over the existing flat junction. “This IS the Letchworth train, ladies and gentlemen. We are being routed via the flyover that has just been opened.” Words of reassurance surely needed as the local train is travelling north on the down slow into open countryside on a track normally used by Peterborough trains. The panic-point for many on board has long passed!

Punctuality gains

The view from the front cab seems to be unchanged at first, but then the diverting route over the flyover comes into view. The gradient is foreshortened making it appear like a switchback ride. It’s an illusion of course, but the rate of climb at 50mph is still impressive. Curving over the main line below, the extensive approach structures come into view followed by the descent on the now poppy-covered embankment to the junction with the existing Cambridge line. Passengers see familiar scenery again and anxiety is allayed. The announcement was right after all. The whole trip takes about a minute longer than via the flat junction at Hitchin, but the gains in overall punctuality will be inexorable.

Curved, skewed and pre-cambered

The new flyover is a classic piece of pipe-dream infrastructure. It’s been something that has been needed for generations. Work finally started in 2011 to a design by Tony Gee and Partners with Hochtief as the main contractor working in alliance with Network Rail.

Overnight work on the new Hitchin flyover [online]

The new 2.3km stretch of railway just north of the town’s station (837 metres of which is on a viaduct) crosses the existing main line and re-joins the old Cambridge route a kilometre to the east. The elevated section is constructed of steel beams with a concrete deck and piers. Whilst simple in concept, this solution presented considerable design and installation challenges as the elevated span is curved, skewed and pre-cambered. It was critical that manufactured steel elements were engineered to fit perfectly and were expertly installed. The viaduct was fabricated by Mabey Bridge in spans of twin plate girders ready to be lifted into position.

Critical main line possession

Mabey Bridge delivered the steelwork to site for assembly alongside the track. Starting with the main span of the viaduct, the 29 pairs of girders were lifted into place. The main span – constructed of a pair of braced beams weighing 300 tonnes – required the use of a 1200-tonne capacity crane.

As we reported in our August 2012 edition, the project programme was saved when a critical main line possession had to be put back a week to accommodate the hordes likely to be travelling to and from a Red Hot Chili Peppers’ rock concert at Knebworth. Saved by a delay? Indeed it was as the original weekend’s weather was dire and would have stopped the critical crane lift. In the end, an astonishing brief period of dead calm conditions occurred the next week and all was well.

Finishing off

So, with the new route open, what else is there to do? As Network Rail’s project manager Nick Hilton said, “It’s just a matter of finishing a few off- track elements and general tidying up. The temporary cabin city has gone and the fairly extensive temporary road junction into the site is being removed. The hill that was ‘borrowed’ for some of the fill material is being regraded and the area is gradually getting back to normal.”

The next Cambridge train pulls into the down platform.

The starter signal is held at red. There’s no feather, no route indication over the flyover for this service because, so far, only a few of the 300+ drivers using the route have gone through their route learning.

It waits. A north-bound main line train passes. It waits. A south-bound train passes. And still it waits. Passengers are getting twitchy.

Another north-bound train thunders through. Finally the route across the existing junction is cleared and the Cambridge train pulls away six minutes late.

The flyover may be a project that has been a long time coming but, at this rate, it will prove its worth very quickly.

Grahame Taylor
Grahame Taylorhttp://therailengineer.com

Structures, railway systems, railway construction, digital data

Grahame Taylor started his railway career as a sandwich course student with British Railways in October 1965, during which he had very wide experience of all aspects of railway civil engineering.

By privatisation, he was in charge of all structural and track maintenance for the Regional Railways’ business in the North West of England.

In 1996, he became an independent consultant, setting up his own company that specialised in the capturing of railway permanent way engineering knowledge using the then-new digital media. As a skilled computer programmer he has developed railway control systems and continues to exploit his detailed knowledge of all railway engineering and operations.

He started to write for Rail Engineer in 2006, and became editor two years later. During this time, he has written over 250 wide-ranging articles and editorials, all the while encouraging the magazine’s more readable style of engineering reporting.


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