By Kate Newman BBC News Online

A �400m project to build an extension to London's Tube network is being delayed - by a once-forgotten piece of the capital's rail heritage.

Kelly Udall has to take a bus and four different Tube trains to get to work everyday.

The 28-year-old lives in Streatham, south London, but works in Wapping, east of the City.

If the East London Line extension was in place his journey time would be halved.

The line will be extended in the north and south to build an "M25" on the railway - part of an Orbirail strategy being developed by the Strategic Rail Authority (SRA) with Transport for London (TfL).

Click here to see the planned northern extension

Local people see the extension as vital new money and jobs for some of London's poorest areas.

But there is a stumbling block in the way in the form of a viaduct.

The project was set to go ahead when English Heritage made a u-turn and said that the "needless demolition of the Bishopsgate goods yard would be a conservation tragedy".

Inside the goods yard is Braithwaite Viaduct, built in 1839, which is one of the earliest examples of railway engineering in the world.

The Braithwaite Viaduct dated back to 1839

Patrick Pugh, assistant director of the London region for English Heritage, said: "A national survey found that the goods yard was much more important then had been previously appreciated.

"It is one of the earliest surviving railway structures in the world."

In March this year the Culture Secretary Tessa Jowell announced the viaduct was a listed building.

Many thought this could mean the end of the planned extension.

'Carry railway'

But English Heritage has said there is a way round the problem.

They commissioned a report from structural engineers in the hope of proving the structure was capable of still carrying rail traffic and being incorporated into the current plans.

But London Underground (LU) is worried about the structure as it suffered a huge fire in the 1960s and has not had any repairs or maintenance for over 40 years.

Exterior of Bishopsgate goods yard

A LU spokesperson said: "Engineers believe that it is not safe to run a railway over these structures."

If they cannot demonstrate to the Railway Inspectorate that the structures are safe then "the extension would not go-ahead".

LU said it only needs to demolish part of the goods yard and that is still the preferred option.

But English Heritage says its research shows a concrete deck could be constructed above the existing goods yard to hold a station.

'Limbo situation'

Mr Pugh said: "We want the extension as much as everyone else.

"This is a practical proposition which could keep the original structure."

Meanwhile the people who the extension is going to help are worried that it is taking too long.

A private company will finance and build the line

Archie Galloway, chairman of the East London Line group which represents leading groups in the capital in support of the extension, said it is a "limbo situation".

He said: "It's important to get something there for London instead of talking and taking more time."

The SRA is in the process of appointing a private company to finance, design, build and maintain the extended line.

Once it is appointed, which will be by the end of this year or early 2003, the project will be handed over.

A LU spokesperson said: "We are still hoping to have it all completed by 2006."

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