# Towards the Internet of Smart Trains:
## 6 IoT-Enabled Services: From More Efficient Operations to New Business Models
### Smart Infrastructure
#### 6.2.1 Advanced Monitoring of Assets  
Sensors can be used to decrease the failure rate and enhance the reliability of trains, signals, and tracks. Such sensors are able to monitor equipment with the objective of generating alerts about the need for attention on critical elements of the train. In this way, costs are decreased and asset usage is optimized by lowering the number of trains taken out of service for inspection, preventive maintenance, or for replacing certain parts after a deficiency is detected. The most common systems to be monitored are represented in Figure 4.  
As it was previously explained in Section 2.6, infrastructure is usually monitored by using WSNs, which are able to assess the condition of tracks, track beds, bridges and the equipment placed on the tracks. Moreover, WSNs can be used to monitor tunnels or to detect intrusions and abandoned items in stations. It is worth mentioning Structural Health Monitoring (SHM), which is currently an essential field for the railway industry [78] and which has been reviewed recently in the literature [79]. Traditionally, SHM systems made use of sensors wired to data acquisition systems, but, thanks to the evolution and the lower cost of wireless devices, in recent years researchers have proposed solutions based on WSNs [80]. A relevant requirement is the need for a precise time synchronization with a resolution of microseconds [81]. This requirement is due to the fact that certain measurements, like vibration monitoring, demand accurate timing and synchronized sensing at high sampling rates [82].  
The reflections derived from the metallic structures of the train also pose a problem for WSN communications due to the multipath effect. This issue can be addressed by modeling the communications channel and then selecting a proper physical layer. Different researches have studied this problem in different SHM scenarios, including bridges [83] and railway tracks [84]. Another example of a WSN deployment at railway tracks to analyze the vibration patterns caused by trains is described in [85]. Moreover, existing monitoring methods are studied in [86], where the authors integrate three methods to monitor rail damage in the turnout (railroad switch) zone (i.e., fiber optic detection systems, optical imaging and Lamb guided wave detection systems).  
WSN-based condition monitoring for the rail industry is the object of the survey presented in [87]. Such a review analyzes the most commonly used sensors for condition monitoring, their configuration and the main network topologies. Another interesting WSN-based system for condition monitoring is proposed in [88], where it is collected data on the infrastructure structural health during the trip to later send the information to a BS. Then, such a BS makes use of the next trains as data mules to upload the information.  
Another real-time WSN-based railway SHM system is presented in [10]. In such an article the researchers focused on designing a customized MAC layer and a synchronization algorithm. According to the results, the sensor nodes that belonged to the same BS presented jitter values within 1 us, while nodes from different BSs had a maximum jitter of 2 us. It is also interesting the work of [PERSON] et al. [89], who describe models and algorithms for optimizing the physical topology of a sensor network aimed at monitoring the condition of the infrastructure.  
A railway bridge is monitored by a WSN-based solution in [90]. In such a system eight nodes collect data that are transmitted to a TmoteSky that acts as BS. The system uses accelerometers to detect when a train approaches and crosses the bridge monitored. The system uses a self-organizing routing protocol whose objective is to make the data reach the BS, which sends them to the RCC through a UMTS transceiver. Other authors proposed similar WSN-based systems for monitoring railway infrastructure either by using Wi-Fi [91] or Zigbee [92].  
Figure 4: Systems usually monitored in a train.