ENGINEERS will lock in the final pieces of a £3m road bridge over a high-speed rail link tonight.

The construction crew working on the 70-metre Relley Bridge, on the outskirts of Durham City, could not use a crane to lift the 450-tonne bridge, because of power cables overhead and the busy East Coast Main Line rail route below the structure.

Instead, they are sliding it into place, using powerful jacks and rollers, at a speed of only 24 metres per hour.

The first phase of the operation was completed successfully on Thursday, but work on the final phase has had to wait until tonight, when there are no trains running.

The existing Relley Bridge, built in 1872, was designed to carry nothing heavier than horse drawn traffic.

These days, it carries the B6302 road over the East Coast Main Line to villages in the Deerness Valley.

It failed to meet current standards when it was checked structurally as part of a national bridge assessment programme, so Durham County Council agreed to replace it.

Current owner Network Rail has made a contribution to the funding.

The old stone-built bridge also had a sub-standard alignment, was too narrow and did not provide for pedestrians or cyclists.

Its replacement will have both a footpath and cycle track and a better road alignment for approaching traffic, plus safety barriers.

Chris Tunstall, the council's deputy chief executive for the environment, said: "By building the new bridge just south of the original, we have been able to keep the old bridge open and minimise disruption to traffic."

Once the new deck is in position, additional work will be necessary before the new bridge is due to open to traffic next September.

The old Relley Bridge is likely to be demolished at Christmas, when, because of fewer trains over the holiday period, county council engineers expect to have more time to carry out the task.