# A Review on the Prospects of Mobile Manipulators for Smart Maintenance of Railway Track
## 3 Robotic Technologies Used in Railway Track Inspection and Maintenance
### Robotic Repair Technology in Railway  
Inspections are important in railway asset management to verify the integrity of assets and to prevent catastrophic failures. For example, in the UK, once a defect is detected on the track, details of the defect are recorded in track geometry reports (TGR). Based on the standards of Network Rail Limited (NR-L2-TRK-001-MOD1), infrastructure maintenance engineers (IMEs) and track maintenance engineers (TME) categorize the defects as immediate action-level (IAL), intervention-level (IL), and alert-level (AL) faults [69]. Both IAL and IL defects require repair without delay, while AL defects can be scheduled. Repair tasks in railways vary, including bolt or clip removing or fastening, grinding rail surface, welding, removing obstacles, demolishing trackside vegetation, fixing overhead electric lines, cleaning snow or leaf from the track, cleaning rolling stock, repairing rolling stock, etc. Most repairs involve humans and specially designed tools and machines. Moreover, adverse weather conditions and out-of-hours work can create hazardous conditions. Some tasks are physically demanding, involving heavy loads that result in the fatiguing of railway workers, posing a risk to their health and safety. Although robots are used in many inspection tasks around the railway network, they are rarely used in any repair task. This section will summarize all the repair robot-related literature, despite focusing only on the track. Even after scrutinizing the literature rigorously, very few examples of repair robots have been found; those discovered are mostly used in cleaning tasks and are related to the repair task of rolling stock. The first attempt to determine the usability of robots in repair tasks was carried out in 1987. After considering 24 technically viable processes in locomotive rebuilding, the researchers concluded that most of the processes were neither economically beneficial nor productive [70]. Just 2 years later, carrying out a separate study in Toronto Subway, [PERSON] and [PERSON] assessed that robots are economically and technologically advantageous for cleaning rolling stock [71]. Four types of cleaning robots were introduced by the East Japan Railway to tackle the problem posed by the lack of cleaning staff. Among the 4 types of self-propelled cleaning robots, 2 types were for stations, and 2 types were for rolling stock, including dust-collector or sweeper types and floor cleaning or scrubber types [72]. Those robots were equipped with ultrasonic sensors and laser sensors for navigation and obstacle avoidance. [PERSON] et al. designed and demonstrated a self-traction model for cleaning the vehicles used for urban mass transit in China. The robot was designed with a mobile platform and a manipulator to save energy and water. Apart from the path planning and control algorithm, the researchers also developed an efficient energy and water management algorithm [73]. [PERSON] et al. discussed the requirements and conceptual design of a rolling stock front cleaning robot [74], and a prototype was built to analyze the cleaning time and force applied at the cleaning head [75]. Apart from cleaning, robots are also used for cutting, welding, grinding, or material handling. Vale, a mining company in Brazil, developed a robot to repair the rail cars used for the mining operation. As shown in Figure 9, the system had multiple manipulators capable of cutting and welding carbon steel [76]. The wheelset of rolling stock, a combination of 2 wheels with a shaft, is very heavy and requires expertise for repair. NSH USA, formerly known as the SIMMONS Machine Tool Corporation, utilized a robot and overhead cranes to altogether remove the human involvement in transporting wheel sets in different workshop locations [77]. A novel technology has been developed in Britain for in situ rail defect repair which is both cost-effective and semi-automated.