## 6 Steps towards Criteria Fulfilment
### Automated Feature Detection and Evaluation
#### 6.1.1 Common Assets in Masonry Railway Tunnels  
An 'asset' denotes any useful or valuable item associated with railway network operation, encompassing employees, track, signalling, buildings, utilities and civils (structures, earthworks) [225]. Tunnel assets fall under civils, with any unprotected structurally significant entity designated a critical element.  
Hidden Critical Element (HCE) is unobservable from at least one side [226]. Locating HCE has presented considerable challenges for Network Rail (UK) in RTSSI. We identify that for current inspection methods, greatest challenge is presented by detection of blind (concealed but disenable) and hidden (concealed and indiscernible) shafts:  
* **Ventilation Shafts**--Hollow columns extending from tunnel crown to the surface. They facilitate air circulation and were originally used to remove material during construction [227].
* **Maintenance Shafts**--To allow simultaneous excavation of multiple faces, many shafts would be sunk along proposed tunnel routes [228]. Typically infilled or converted to ventilationshafts, capping frequently conceals them for aesthetics (processes rarely recorded in writing). Being unreinforced, many have deformed or partially collapsed.  
This is most true for Wales and Western Regions of the UK, following Network Rail's failure 'to deliver on a commitment to identify all hidden tunnel shafts by the end of 2016-2017' [229] and more recent delays tackling HCE examination schedules in 2019-2020 due to pandemic impacts [230].  
As cavities are prime sources of water infiltration, failure to identify hidden shafts significantly increases risk of accelerated compromise to surrounding structure. Therefore, innovations towards hidden shaft detection present opportunity for highest new research impact in RTSSI.  
Training detectors to discriminate hidden shafts from other features motivates continued summary of other common masonry railway tunnel assets:  
* **Overhead Line Equipment (OLE)**--Furthermore, dubbed 'traction wires' or the 'catenary', these high voltage electrical pickup lines power electric locomotives via onboard pantograph connectors. Systems can be integrated into older tunnels during electrification works. Forms include tensioned metallic cables mounted to the crown and the Rigid Overhead Conductor Rail System (ROCS) [231], which provide more efficient operation in low-clearance tunnels. Structural weakening can result from necessary drilling during install, whilst strong electromagnetic fields generated by the power feed can interfere with data acquisition systems and present line of sight obstruction during haunch or crown inspections.
* **Portals**--Reinforced surfaces surrounding tunnel entrances combating outward deformation induced by shear stresses from continuous shifting of landmass encircling the tunnel [232]. Exposed to the elements, portal rigidity deteriorates, risking collapse if cracks and displacement are not detected early. Reinforcement schemes include buttresses, ground anchors, and steel mesh coverage fixed with soil nails [233].
* **Refuges--**Small arched recesses within the tunnel lining to protect railway workers from locomotives.
* **Buried Utilities--**These can include both metallic and plastic water drainage pipes [234], electrical wiring and telecom cables.
* **Trackside Objects--**These include signage, signals, electrical junction boxes and CCTV units.
* **Culverts--**Small passages allowing watercourses to pass under railway tracks, including underground rivers [235]. Old masonry culverts particularly can be weakened by solution and hydraulic action resulting in partial section collapse, deforming the railbed above and causing water backlog which floods tunnels.