A tunnel is a particularly long type of Bridge (Overline). For more information, please refer to the related article "What are railway tunnels and why are they necessary?".
For the purposes of the SCRCA Project, the 'tunnel' is the hidden structure that lies between a pair of Tunnel Mouths. The interiors of tunnels are not normally visible from outside the railway perimeter (or from inside a moving passenger train), so they cannot be 'assessed' or photographed by our volunteers. Also, in terms of the management of the Conservation Area, their structural form and physical appearance are irrelevant, so they have not been described or classified. However, some information regarding construction methods, maintenance, incidents, etc. may be available in 'snippets' associated with individual tunnels.
See also the related structure type Access Shaft (aka air shaft / ventilation shaft).