This paper presents a case study on the subgrade settlements of four national railway lines induced by twin shield tunnel excavations. The development characteristics of subgrade settlements can be divided into three stages. The monitoring data indicate that subgrade settlements increase gradually before the shield machine crossing the monitoring region, then increase sharply during the shield machine crossing, and finally approach to be stable after the shield machine crossing. Immediately splicing the segments into rings, implementing the compensation and secondary grouting facilitate the control of subgrade settlements. Operational parameters play a crucial role in controlling the subgrade settlements. The effects of the operational parameters of the shield machine on the subgrade settlements are analyzed. The investigation reveals that excessive shield driving velocity causes large subgrade settlements, accompanied by the sudden reduction of rotation torque and total thrust. The excavated earth volume affects the subgrade settlements insensitively because of its easily controlling. Synchronous grouting volume can rise the existing subgrade if not controlled appropriately. Great subgrade settlements cause large earth pressures to be exerted at the chamber. Some valuable references and guidance are given through this paper for the operational parameter setting of shield machines and deformation control of multiple railway lines due to the shield tunnel excavation beneath.