Even a small amount of settlement is critical in the operation of high-speed railways. Recently, in Korea, additional settlement was observed at many sites of high-speed railways after the construction of concrete tracks including sites where ground improvement methods were applied. Among 1,118 observing sites, 95 sites experienced the settlement exceeding the control limit of high-speed railways. The present work described factors causing additional settlement in high-speed railways. Geological features, embankment height, layer thickness, ground improvement method, groundwater drawdown, compaction of frozen soil, and dynamic loading by train operation were selected as the potential factors of the additional settlement. Analysis results showed that high settlement was measured in concrete tracks that were constructed over alluvium and biotite granodiorite layers (85.76%) and the settlement increased according to the increase of the embankment height and the thickness of the subgrade layer. In 92.63% of high settled sites, a sudden increase of settlement caused by groundwater drawdown was observed during the dry season as well. Also, compaction of the embankment during winter and dynamic loading by train operation increased the settlement to a certain degree.