To address the technical challenges in widening existing embankment due to terrain constraints, a new composite supporting structure termed as adjacent composite pile-sheet wall (ACPSW) is proposed, i.e., the new pile-sheet walls are installed side-by-side in the middle of the old pile-sheet walls. Based on the Lanzhou Hub project of Zhongwei to Lanzhou Railway, this paper investigates the pile horizontal deformation, pile bending moment, soil pressure behind the pile and sheet, as well as load-sharing ratio between the old and new pile of ACPSW at different construction stages through field tests and numerical simulations. The results obtained from the field tests were compared with that obtained from the numerical simulation to validate the reliability of the numerical model. Moreover, a serviceability assessment on old pile of ACPSW is also conducted. The results indicate that the new pile-sheet wall and the old pile-sheet wall can deform synergistically and bear the external loads together under new widening embankment loads and train loads, and the load-sharing ratio between old and new pile is 0.62:1.0. The research results can provide a reference for the design and construction of existing line reconstruction and new projects adjacent to existing lines.