Abstract: Intensive dynamic loading is generated when a train is running on railway due to the uneven track. Accordingly, intensive dynamic response occurs in the roadbed and foundation of railway. The excessive vibration response of roadbed and foundation is harmful for the security of running train, and also would make passengers feel uncomfortable. Furthermore, it also would bring harmful vibration to surrounding buildings, and noise pollution to people living around. Therefore, it is practically meaningful to investigate the dynamic response of roadbed and foundation of railways. This paper employs the Biot's dynamic equation as the governing equation, and takes the FEM package FSSI-CAS 2D as the numerical tool. The dynamics of a sandy foundation with ground-water under train running loading is comprehensively investigated. The consideration of the unsaturated zone in the foundation over the static ground-water line is a special point in this investigation. The numerical results indicate that the railway foundation system intensively responds to the running loading of train. Among them, the vertical dynamics with low frequency is the dominant part. It is also observed that the dynamic response of roadbed and foundation of railway both damps in horizontal and vertical direction. The negative pore pressure in unsaturated zone keeps unchanged due to the minor relative displacement of soil particles. The methods, tool and computational cases in this paper can be a research example for evaluating the safety, travelling comfort of a train running on a sandy foundation with ground-water.