This study investigates the response of a circular tunnel embedded in saturated potentially liquefiable ground under moving train loading, with particular focus on the effect of an annular rubber-soil mixture cushion placed around the tunnel lining. A three-dimensional finite element model was developed in OpenSees using coupled solid-fluid soil elements and shell elements for the tunnel lining. Two configurations were analysed under identical soil, boundary, and loading conditions. The first configuration represents the reference tunnel without the cushion, while the second represents the tunnel surrounded by the rubber-soil mixture cushion. The results show that the cushion substantially modifies the tunnel-soil interaction mechanism. The peak lining internal force decreased by approximately 62%, and the force distribution along the lining became more uniform. However, this structural benefit was accompanied by larger tunnel movement and surface settlement. The train-induced invert displacement increased from 15.7 to 56.8 mm, while the maximum surface settlement increased from 19.7 to 25.5 mm. The additional movement was mainly associated with rigid-body settlement, since tunnel distortion remained nearly unchanged. The excess pore pressure ratio remained close to 0.08, indicating that liquefaction was not triggered under the moving train load considered. The cushion also reduced the computed high-frequency acceleration amplitudes by 49% at the tunnel invert and by 65% at both the ground surface and the liquefiable zone. A functionality-based interpretation showed that the structural comparative functionality indicator (CFI) increased from 0.50 to 0.81, whereas the settlement-based serviceability CFI decreased from 0.91 to 0.68. The findings indicate that an annular rubber-soil mixture cushion acts primarily as a lining protection and response filtering measure rather than as a settlement control measure.