## 1 Introduction  
Accidents between trains and vehicles at train crossings are mainly caused by a driver's lack of cognition of an oncoming train. Most of these accidents happen near railroad-grade crossings. Intelligent transportation systems (ITSs) will improve the efficiency and safety of transportation by leveraging advanced wireless communications and sensor technologies and systems. The Operation Lifesaver project adapted from vehicular ad-hoc networks (VANETs) demonstrated that an early warning system could significantly improve railroad safety by avoiding vehicle collisions [1]. The vehicular communication system is the dominant system used in VANETs and ITSs [2]. It has two different forms to provide services: vehicle-to-vehicle (V2V) and vehicle-to-infrastructure (V2I) communications. The vehicular communication system operates in the 5.9 GHz ITS band: 5.850-5.925 GHz. The authors of [3; 4; 5] introduce the proposed vehicle-to-train (V2T) early warning system and present their Dedicated Short-Ranged Communication (DSRC)-aided system performance and propagation channel measurements. The DSRC is the primary protocol used for the vehicular communication system, which can be found as IEEE 802.11p. The authors of [5] present the performance of the DSRC warning system with the performance metric of PER in the same topology as the conducted channel measurement that is presented in this paper. These papers demonstrated that DSRC technology has the potential to offer a cost-effective approach for the deployment of an early warning system to avoid train-to-vehicle collisions and other railroad accidents. The infrastructure near crossings may act as a relay to provide sufficiently reliable warning signals. In [3; 4; 5], the early warning system architecture using infrastructure near the crossing is presented. From the results, the wireless system using the 5.9 GHz frequency band can potentially be used for train-to-infrastructure warning applications. The communications range of the vehicular communication system is sensitive to the operating environment and significantly degrades in non-line-of-sight (NLoS) channel conditions, which are observed at many railroad crossings [6]. Prior work has reported channel statistics for vehicular environments with channel statistics that can affect intra-packet channel fluctuations for large packet sizes [7; 8; 9; 10; 11; 12]. Thus, knowledge of grade crossing propagation channel environment statistics will allow for the development of flexible yet robust communication protocols that can operate even under potentially degraded conditions; therefore, a presentation about generalized statistical channel conditions can provide an expectation of a system performance that will be implemented at a train track. The studies of the propagation channels for vehicle-related environments were performed in several previous studies. Most of the studies considered V2I or V2V [13; 14; 15; 16; 17; 18]. Only a few previous studies considered Train-to-Train (T2T) [19; 20] or Train-to-Infrastructure (T2I) [21] environments. Interestingly, their interested frequency bands were different: 2.4 GHz [22], 2.1-2.6 GHz [21], 5.2 GHz [19], 5.9 GHz [14; 15; 16], etc. The applications that were evaluated at 5.9 GHz were V2V or V2I. The propagation channel evaluations at the environments were expected to use T2T or T2I were not using the 5.9 GHz frequency band, which is the operating band by the vehicular communication protocol. We observed from the related research surveys that studies on channel models and the measurements for the relevant T2I at 5.9 GHz frequency band scenarios are scarce. Therefore, we conducted the 5.9 GHz T2I propagation channel measurements at real operating train crossings and analyzed the propagation channel characteristics under these scenarios. This paper presents (1) T2I channel measurements at the 5.9 GHz frequency band and (2) the propagation channel data analyses at real operating railroad tracks in the United States.