**2.3 Other OFS technologies**

The probable future technologies in the railway sector related to OFS are FBGs and DAS, as discussed. Other technologies linked with Brillouin scattering and Raman scattering have been investigated quite often in the railway sector. The reason is that


#### **Table 3.**

*Train traffic management applications of DAS in refs to literature.*


### **Table 4.**

*Train traffic management applications of DAS in references to previous work.*

the technologies related to Brillouin-based scattering cannot differentiate between static sensings such as stress and temperature. An abrupt change in the temperature occurs after the train traverses the railway track, which provides confusion between the real measured variable (stress) and temperature. Though some efforts are made in this regard to adopt the Brillouin-based sensing technologies in railways such as traffic monitoring [64], railway infrastructure [65], track deformations monitoring [66], and operation monitoring of the railway infrastructure [67]. However, the mentioned research work is insufficient for these technologies to be adopted in the railway sector anytime soon. The reason the OFS technologies related to Raman scattering are not used in the railway sector is because of their use in temperature sensing only. A longdistance infrastructure monitoring with a temperature sensing-only application is not the cost-effective solution, and hence this sensing technology has not been

investigated so far in this sector. Beside, the interferometric-based OFS cannot be used in the railway sector due to the nonlinear nature of this sensing, as well as the non-distributed nature. Though, [68, 69] have presented their work in interferometric-based sensing, but these techniques were not verified with a distributed or quasi-distributed sensing techniques over a very long range of railway track or vehicle. OFDR is another type of interferometric-based sensing technology with distributed nature of its sensing. However, due to its short-range applications, this type of interferometric-based sensing cannot be used in railway applications.
