This paper presents an empirical method and a modified cross-section racking deformation (MCSRD) method to assess the seismic capacities of the old Sanyi railway tunnels that were constructed about one hundred years ago. The proposed MCSRD method is simple and fast. It can automatically take into account the interaction between underground structure and the surrounding ground under seismic action, by using a 2-D finite difference method. The seismic capacities based on the MCSRD method are higher than those based on the empirical method by one JMA intensity scale. The field monitored performance of these tunnels during the 1935 Hsinchu-Taichung and 1999 Chi-Chi earthquakes agree very well with the seismic capacities assessed by the MCSRD method.