The current work on the railway tunnel beneath Southampton shines a light on a unique part of the city's industrial heritage.
The 483m (528yd) tunnel starts at Southampton Central station and runs under the city centre parks, emerging beyond East Park Terrace.
Now work has begun to lower the tunnel floor to accommodate a new generation of larger freight trains.
But work on the first tunnel under the centre of Southampton started more than 200 years ago - for a waterway.
Hundreds of trains use Southampton tunnel every day
In the early 19th Century, the short-lived Salisbury to Southampton canal was constructed.
Initially the canal stopped at Redbridge, but there was a proposal to extend the waterway into Southampton.
The plans were drawn up and a tunnel under the central area of the city was partly dug. But the project was never completed and was abandoned in 1808.
By the middle of the 19th Century, Britain was embracing the industrial revolution and railways were being built across the country.
A wealthy West Country solicitor, Charles Castleman, proposed a link between Southampton and Dorchester.
In 1845, Parliament approved construction of the line which would serve Southampton, Ringwood and Dorchester. The line was to run from Northam Junction and through the town centre.
Heading west, the line hugged the coast along the western shore of Southampton Water following the old canal towards Redbridge and on through the New Forest before crossing the county border into Dorset.
483 meters (528 yd) long
Walls made of brickwork 2ft - 3ft (0.6m - 0.9m) thick
3rd rail electrified at 650V
40mph (64kph) line speed
Originally built adjacent to the docks
Initially, the plan was to build a station close to Royal Pier. Plans were changed and Blechynden station was built, close to what is now Southampton Central.
But the hill at Marlands stood between Blechynden and the proposed link to the London Line at Southampton Terminus stations (outside the old South Western Hotel).
As local historian Jake Simpkin told BBC Hampshire, the engineers had just one option when planning their route.
"Going surface was dismissed because it would destroy the central parks then being laid out," he said, "so the plan was to take the line under the town".
The tunnel was planned to partly follow the line of the intended canal route.
Civil engineer Samuel Morton Peto was commissioned to construct the tunnel.
Peto was a highly experienced contractor, having laid railways in England, Russia, Norway, Algeria and Australia.
Working with a number of different business partners, he was responsible for constructing many of the buildings and routes of the developing railway network.
Jake Simpkin is a Southampton tour guide
Notably, his company built two stations in Birmingham as well as sections of the Great Western Railway.
As a partner in Peto and Grissell, he was also responsible for many structures in London including Nelson's Column.
Miners from Cornwall were drafted in and construction on the tunnel under Southampton began in April 1846. But even with their specialist skills, the works were far from straight forward.
"Most of the tunnel was built as an open work and then capped in with a brick vault" explained Jake.
"The only length of actual tunnelling was under the High Street".
The tunnel opened in August 1847
Shortly before it was due to open, flooding of the old canal section of the route caused part of the tunnel to collapse.
The railway was completed in June 1847 but because of delays in construction the tunnel did not open until August of that year.
For the first four months, passengers were taken across the town by horse drawn omnibuses - showing that replacement bus services are nothing new.
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