Severn Tunnel: From its creation to electrification

Work to install a customised electrified roof track has started in the Severn Tunnel as part of the electrification of the south Wales mainline.

The Swiss-made device is so advanced Network Rail has set up its own engineering course to train contractors how to install it.

The work will eventually lead to a complete closure of the tunnel in September for six weeks.

Diversions will go via Gloucester.

BBC Wales takes a look at its timeline, from creation to electrification.