fkw - inside

View from inside Rough Castle Tunnel, looking northwards to the aqueduct and the boat lift.

I noticed that the interior of the tunnel is "shotcreted" - a layer of concrete that has been "spray-painted" - this technique is often used to stabilise the interior surface of a new tunnel, to prevent spalling, amongst other potential problems.
{I'm familiar with the method from it being used at another tunnel know quite well, one in North Wales}