# Rail Diagnostics Based on Ultrasonic Guided Waves: An Overview
## 10 Discussion and Conclusions
### Advantages and Disadvantages of the Considered Systems  
The systems reviewed in this manuscript present both advantages and disadvantages. Due to the approaches followed being so different, a direct comparison is impossible. However, an attempt can be made to assess their pros and cons by considering their main characteristics, range of application, and ease of development. The main disadvantage of the active on-board inspection system is its limited inspection speed; in these terms, the system does not achieve a significant improvement over the conventional ultrasonic inspection systems. On the contrary, if the passive on-board system is considered, the opposite problem is found: in fact, the system is able to provide good results only when the diagnostic vehicle is moving at high speed or along curved tracks. This limitation can be circumvented by exploiting the ability of this system to reconstruct the waveguide transfer function (which allows defect detection) without the perfect knowledge of the excitation signal. An alternative to on-board approaches is constituted by land-based systems, such as the UBRD and the UBRD evolved to recognize small-scale defects. The main feature of the simple UBRD system, the detection of complete rail breaks, is already implemented in the track circuits, which are well known to railway engineers. The advantage of the UBRD system is that it does not perform critical operations in terms of railway traffic safety (such as train detection); therefore, it might not be subject to stringent safety constraints, unlike track circuits. The main advantages of the evolved UBRD system are represented by its ability to identify defects before a complete rail breakage and the possibility of accomplishing a continuous monitoring of the health status of rails. The last feature is not provided by on-board systems that, because of their nature, call for a periodic inspection of the infrastructure. It should be also kept in mind that a continuous monitoring system allows for acquiring a large number of measurements having limited accuracy. In fact, the process of searching for a defect, identifying it, and monitoring its evolution is repeated continuously over a long period. Basically, it does not matter that a defect is identified and reported at its onset, when it still has a small size. What really matters is that the presence of a defect is signaled and its evolution kept under control over time, so that maintenance operations for its removal can be planned before they become dangerous for the safety of railway traffic. A periodic inspection system, depending on the frequency of its tests, needs to acquire accurate measurements. A defect must be detected in time, when it is still small in size, so that its evolution can be monitored, inspection after inspection, before it reaches a critical state. This is needed since any defect can exhibit a sudden evolution. Other advantages and disadvantages to be taken into consideration are those related to the practical usage of the considered systems. If the health of the rails of an entire railway network has to be monitored, a land-based system must be installed on all its tracks; this could be time-consuming, laborious, and expensive. In addition, system maintenance might require a relevant effort, especially when some of its devices have been installed in remote areas, which are hard to reach. Another relevant problem is represented by the fact that its equipment, being physically attached to the rails, could represent an obstacle during any operation of infrastructure renovation. On the contrary, if an on-board system is employed, it is only needed to equip an adequate number of diagnostic vehicles, and make them travel frequently and regularly throughout the network. This means that fewer devices, albeit more complicated, need to be built, installed, and maintained. Moreover, their regular maintenance may take place in workshops and not on the field, perhaps in remote areas.