As the requirements for reduced vibration of URT viaduct lines continue to increase, low-stiffness structures have been used extensively. When the TSI are adversely affected by low-stiffness structures, the TGI maintenance workload increases sharply. Therefore, it is necessary to predict and evaluate the TDGS of low-stiffness URT viaduct to resolve the conflict between vibration-reduction design and track maintenance. First, the train-steel spring floating slab track-U-beam viaduct model is established. Second, virtual track inspection method is used to obtain the virtual dynamic irregularities. After comparing with actual measurement results, we investigated the influence of linetype, bridge span, and static irregularities on the security indexes and the TDGS. The results showed that for curved viaduct, in addition to vertical dynamic irregularities, alignment/horizontal dynamic irregularities also have characteristic wavelengths at 1/n times bridge span. The time-domain peak values were larger than those of the straight viaduct. Moreover, as the bridge span increased, the TDGS degradation of the curve viaduct was more significant than straight viaduct. The vehicle security indexes of the low-stiffness viaduct lines have large safety margin, while TDGS indexes are close or even exceed the limit when structures are improperly designed. This could bring insurmountable difficulties to the track maintenance.