This paper presents a special case study on the performance of geosynthetics-reinforced and pile-supported embankments (GRPS) over silty clay of medium compressibility. A full-scale high-speed railway embankment with four instrumented subsections over medium silty clay was constructed in three stages. Three of the sections were reinforced by geosynthetics and cement-fly ash-gravel (CFG) piles with different configurations, while the other section was only reinforced by geosynthetics, which was used as a reference. The discussion mainly focused on the deformation behaviors of test embankments. The results show that (1) both the vertical settlements and the lateral displacements slightly increased until a certain fill height. Afterwards, they increased rapidly due to the transformation from the over-consolidated state to partly or entirely normally consolidated state of the foundation soil. (2) CFG pile reinforcement performed well in terms of reducing settlements and lateral displacements. (3) The lateral displacement is a useful indicator for evaluating external stability of GRPS embankments. Moreover, the comparison of soil stiffness obtained from in situ and laboratory suggests that an appropriate equivalent modulus may be obtained based on the stiffness-strain curve from a triaxial test with local strain measurements while considering the correct strain level obtained from field measurements of vertical settlements.