Every tunnel within the SCRCA has a masonry-built portal or 'Tunnel Mouth' at each end. These structures form the entrances to / exits from the tunnel and they are designed to prevent landslips and rockfalls that could foul the running-lines (railway tracks) and cause an accident. Some 'Tunnel Mouth' structures include drainage channels to divert surface water away from the railway lines, thereby reducing the risk of flooding. Examples of the latter include Blea Moor Tunnel North Portal and Shotlock Tunnel South Portal.
For more information, please refer to the related article "What are railway tunnels and why are they necessary?".