Train: Why Severn Tunnel links beer, Scuba diving and Oasis

Water-collecting drains have been added into the crown of the tunnel to minimise saltwater dripping on the tracks.

Ventilation fans have also been installed to create a constant breeze to help make the environment kinder to the steel track and electric wires.

Each of the 66 lengths of new railway track that will be installed this July is bigger than two football pitches at 215m (705ft) long and cost about £14,000 apiece.

Some 10,800 concrete sleepers, costing £90 each, will also be swapped.

“There are sceptics saying you’re blocking the railway again,” added Mr Paynter from Network Rail.

“But I’m doing it for the greater good to ensure a resilient and safer network. This is a temporary period of disruption, but if we don’t replace the track, everything becomes life-expired. It’s short pain for long gain.”