The braking system of a high-speed train has a crucial role for the safety of human mass transportation. However, it is hard to acquire design parameters of the braking system in the development stage of a new high-speed train. In this paper, we build a Hardware-In-the-Loop (HIL) system for the braking system of the Korean High-Speed Train (KHST) that is supposed to reach 350 km/h, and analyze the characteristics of the braking system of KHST (composed of 7 cars) via real-time simulations. In the HIL system that is built using a DSP board of dSPACE, the dynamics of the 7 car bodies and several bogies and characteristics of connecting devices between cars are considered individually. Simulation results show that the designed braking system of KHST is valid and satisfies design specifications.