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Method of sliding mesh and segregated compressible large eddy simulation was used to simulate the flow field of a 1/25 scale eight-coach China Railway train running from open air into long tunnel, validated by the same scale Intercity Express 2 (ICE2) train. Side force fluctuation increases greatly when train enters tunnel. Short-time Fourier transform on side force coefficients of Coach 5 and 8 presents the peak frequency at about 95Hz. The hunting motion of laterally swinging airflow in lower space near the train is analyzed. Correlation analyses of pressure time histories reflect its traveling mode. Propagating velocity is 57–62% in open air, and 75–80% in tunnel of the operating speed. Wavelength is both in the range of 0.3–0.6m. Spectral Proper Orthogonal Decomposition (SPOD) and POD extract the transverse pattern of hunting motion. Total POD energy increases when hunting motion appears. Consistent to side force spectrums, SPOD energy spectrums present peak frequencies at 95Hz, where energy is larger in tunnel than open air. Mode 1 of POD and SPOD under peak frequencies presents similar spatial distribution: anti-symmetrical streamwise velocity fluctuation, vortex pairs outside bogie cavities and large-scale circulation. The relation between coherent structures and side forces is well established by SPOD. • Aerodynamic forces and flow patterns of train from open air into tunnel are numerically investigated. • Transverse pattern of hunting motion is extracted by Spectral Proper Orthogonal Decomposition (SPOD) and POD. • The relation between side forces and hunting motion is established by SPOD. • Aerodynamic forces in transition section are analyzed by sliding average and short-time Fourier transform. [ABSTRACT FROM AUTHOR]