## 2 Related Work  
Trains are considered as part of the intelligent transportation system and multiple applications are enabled by smart and connected vehicles in the broader sense. Regarding the safety applications using wireless protocols, reference [23] explores the antenna basis for a train crossing and derives an optimal antenna pattern for practical usage. Moreover, references [3; 4; 5] presents the DSRC, a vehicular communication system protocol that uses the 5.9 GHz frequency band, which can provide sufficient safety for T2I scenarios and the potential for using the vehicular communication system for the train-safety application. Since the vehicular communication protocol uses a 5.9 GHz frequency band, studying the 5.9 GHz frequency band for T2I scenarios can help future T2I system simulations or in the performance expectation before implementing T2I communication systems.  
The propagation channel measurements are conducted in various frequency bands, [24] evaluated in the Terahertz band, [25] evaluated in the 2.6 GHz band, [19] evaluated in the 5.2 GHz band, [21] evaluated in the 2.1-2.6 GHz band, and [22] evaluated in the 2.4 GHz band. These studies considered vehicle-involved scenarios with different communication system protocols than the vehicular communication system, which uses the 5.9 GHz frequency band.  
The propagation channel characteristics for the 5.9 GHz vehicular communications environment are presented in [7; 8; 9; 10; 13; 14; 15; 16; 17]. The authors compared the results between time- and the frequency-dependent K-factor in [7] and the RMS delay in [8]. Similarly, references [9; 10] presented the delay spread in environmental scenarios, such as rural, urban, and highways for the 5.9 GHz channel. References [15; 16] evaluated the V2V propagation channel with characteristics of both the RMS delay and K-factor. References [13; 14; 17; 18] conducted measurements for V2I scenarios. Reference [13] presented the path loss exponent and the RMS delay spread for a 350 m long road. Reference [14] presented the path loss exponent, K-factor, and RMS delay spread in a 120 m long underground environment. Reference [17] presented both the Doppler shift and RMS delay spread in an expressway. Reference [18] presented only a path loss for the tunnel environment. References [10; 26] used the power delay profile (PDP), which we leveraged for our data analysis.  
Even though many propagation channel evaluation studies for both the V2V and V2I have been performed, only a few studies are interested in train-track environments. References [19; 20; 21; 26] conducted measurements with trains or near train tracks. Reference [26] used measurements to derive the empirical models for the 5.8 GHz propagation channel and railroad environments. Reference [19] conducted measurements for the T2T scenario with 5.2 GHz frequency bands. Additionally, the authors present the tapped delay line channel models and compared them with the 802.11 bd simulation data. In [20], a T2T channel sounding measurement in the 5.9 GHz frequency band was presented. Reference [21] presented channel models at railways considering T2I scenarios but the selected frequency band was 2.1-2.6 GHz.  
As we observed from the previous studies, most studies evaluating the 5.9 GHz band used V2V/V2I scenarios. Moreover, the propagation channel measurements and evaluations with trains or near train tracks were not performed. The categorization of previous studies is shown in Table 1. The previous studies were more likely to have measured in different frequency bands than the 5.9 GHz band. Therefore, our 5.9 GHz T2I channel measurement is rare and more directly related to the vehicular communication system protocol, which uses the 5.9 GHz frequency band.