This paper presents a comprehensive study on millimeter-wave-based mobile hotspot network (MHN) system for high-speed train (HST) communications, including system design, field trial, channel modeling based on measurement campaign, simulation, and validation. First, an overview of the MHN system and the design of an MHN enhanced (MHN-E) system including physical layer specification and single frequency network are provided. Second, the overall design of prototypes for MHN conventional (MHN-C) and MHN-E systems is presented, and then the recent experimental results of a field trial conducted at Seoul subway line 8 with the MHN-C prototype and demonstrations with the MHN-E prototype are analyzed. Third, this paper presents a measurement campaign investigating channel characteristics of a rural HST environment and a QuaDRiGa-based channel model developed based on the parameters extracted from the measurement. Finally, a computer simulation is conducted to validate the performance of the MHN-E system at high mobility of up to 500 km/h under the QuaDRiGa channel model, which suggests a good methodology for the validation of HST communication systems at the highmobility since it is impractical to be tested by real testbeds as of now. The simulation results revealed that the MHN-E system is capable of providing a broadband mobile wireless backhaul with a peak data rate exceeding 5 Gb/s at a speed of 500 km/h.