IT cost £10million and resulted in thousands of rail users enduring lengthy detours or switching to buses while it was completed.

But work to prepare the Severn Tunnel for electrification - as part of a £2.8 billion programme for the whole of the South Wales-London main line - is now complete.

Trains are running through the tunnel again after a six-week period of complete closure that was deemed necessary in order to dramatically shorten the amount of time the preparation took.

Without such a closure, which meant bus instead of train journeys for some, and 35-minute rail detours through Gloucester for others, the work would have taken an estimated five years.

But what went on inside the tunnel during the closure?

This timelapse video from Network Rail, which can be viewed here, shows that the tunnel was a hive of activity during what was the longest continuous period of closure during its 130-year history.

A small army of Network Rail engineers worked around the clock along the tunnel’s 7,000-metres (4.3 miles) length, completing the equivalent of 18,000 days of work. Three teams of 200 engineers worked shifts around the clock.

Perhaps the dirtiest part of the project was completed before the preparation began in earnest, as more than 40 tonnes of soot was removed from the tunnel walls.

More than 20 conductor rail anchors and 240 metres of conductor rail were installed in the tunnel, per 10-hour shift, and 7,000 anchors were installed to support the new electric equipment.

Fourteen kilometres of overhead conductor rail were also installed, and these are supported by close to 1,650 pieces of equipment called drop tubes, which are suspended from the top of the tunnel.

All of this specialist equipment will help deliver quicker journey times - of up to 21 minutes between Swansea and London Paddington - and an increase of up to a quarter in the number of seats per train when new rolling stock is introduced.

Network Rail currently expects to complete the electrification of the line between London Paddington and Cardiff by 2019.