This study experimentally investigated the long-term deformation behavior of a ballasted railway turnout affected by adjacent deep excavation through an integrated multi-sensor field-monitoring approach. The monitored facility consisted of two 60 kg, #10 single turnouts located approximately 13 m from an excavation with a maximum depth of approximately 21 m. Field monitoring was conducted for approximately 640 days, from the pre-excavation stage to the completion of the underground structure. Three complementary measurement methods were employed: a track master system for track geometry, tilt-angle-based rail displacement sensors for local rail displacement, and an automatic leveling system for absolute sleeper displacement. The relative measurement methods primarily identified local rail geometry variations within approximately ±1–2 mm, whereas the absolute sleeper measurements revealed substantially larger support system movements. In particular, a maximum sleeper settlement of approximately −9.1 mm and a maximum heave of approximately +4.9 mm were observed at different locations of the T2 turnout, producing a longitudinal differential displacement of approximately 14 mm within the turnout system. The results indicate that excavation-induced turnout deformation consists of both global support system movement and local differential rail deformation, which cannot be completely characterized using a single measurement reference framework. The principal contribution of this study is the experimental clarification of the complementary roles of relative and absolute displacement measurements in distinguishing local track geometry changes from global sleeper support movement. The proposed integrated monitoring framework provides a practical basis for identifying critical deformation zones and improving the structural assessment and maintenance of turnouts affected by adjacent excavation.