The heterogeneous network environment in railway Internet of Things (IoT) networking increases the complexity of network hierarchy “macro-small-micro” multilayer fusion of spectrum resources, and the fast mobility condition in the high-speed railway wireless communication exacerbates the correlation and dynamics of, “time-space-frequency,” multi-domain coupling of spectrum resources. This article fully considers the impact of high-speed mobile conditions and heterogeneous network environments. It also reveals the underlying cause of the poor performance of wireless spectrum opportunistic access of high-speed railways caused by the lack of capabilities in the electromagnetic spectrum space cognition, and proposes our multi-domain/multi-granularity/high dynamic electromagnetic spectrum space cognitive theory, based on spectrum situation suitable for future high-speed railways. The article specifies the feasibility, operability, and effectiveness of the spectral situation awareness theory in high-speed heterogeneous environments.