Aiming at the problem that high-speed train axles are continuously subjected to alternating loads under complex service conditions, and existing monitoring devices suffer from bottlenecks such as geometric mismatch and cross-crosstalk of multidimensional signals, this article proposes a dual-side isolated topological multimode flexible sensor. The innovative hardware architecture effectively suppresses signal crosstalk and realizes real-time monitoring of strain, temperature, and vibration of train axles. With cross-shaped polyimide serving as the electrical and thermal insulation barrier, the top layer is spin-coated with a polyvinylidene fluoride piezoelectric composite film doped with boron nitride nanosheets and barium titanate particles, enabling broadband real-time acquisition of vibration parameters. Based on patterned screen printing and other fabrication processes, the bottom layer coplanarly integrates strain units with liquid metal and polyvinyl alcohol crosslinked networks, as well as serpentine silver paste temperature units with excellent in-plane tensile compliance. The entire device is encapsulated with polyimide, and a thermoplastic polyurethane/boron nitride composite film with high out-of-plane thermal conductivity is locally integrated in the bottom temperature measurement area. This structural design establishes an orthogonal decoupling mechanism for force, thermal, and vibration signals as well as efficient heat transfer pathways at the physical topological level. Performance evaluations demonstrate that the sensor achieves a vibration sensitivity of 0.404 V/N within the frequency band of 10~50 Hz. The strain-sensitive layer delivers a maximum gauge factor of 32.11 and a low response delay of 26 ms, enabling vector decoupling capability for two-dimensional alternating stress fields. The temperature unit achieves a resolution of 0.5℃ within a wide temperature range of 0~120℃, while the strain crosstalk interference rate remains lower than 0.3% under 8% mechanical stretching. The proposed dual-side isolated topological structure can effectively suppress physical parameter crosstalk in multimode monitoring without relying on complex back-end compensation algorithms. Verified by bench simulation experiments, the sensor realizes high-fidelity, synchronous, and independent extraction of multiple physical quantities under strongly coupled operating conditions, providing reliable data support for in-situ condition assessment of rotating components of high-speed trains.