Abstract: The service reliability of the locking mechanism, as a key component of variable-gauge bogie, directly affects the operation safety of the variable-gauge high-speed train. Surface engineering protection was experimentally investigated in this work to solve the fretting damage issue of the locking mechanism in actual service. The test substrate (35CrMo alloy steel) was subjected to plasma nitriding treatment and performance characterization on the basis of the principle of surface strengthening against fretting damage. The tangential fretting wear tests under dry and grease lubrication conditions were performed using a self-developed modular multi-functional fretting wear test system. Three displacement amplitudes of 10, 30, and 60 μm were implemented in the tests, the normal load was maintained at 20 N, and 5 000 test cycles were performed at a frequency of 2 Hz. Subsequently, the surface morphologies and tribo-chemical state of the fretting wear region were comprehensively analyzed by scanning electron microscopy, white-light interferometry and X-ray photoelectron spectroscopy. The wear behavior, energy dissipation, damage mechanism of the substrate and plasma-nitrided surface under different working conditions were revealed. Results indicate that ε-Fe_2-3N and γ'-Fe₄N phases form a dense compound layer of about 8 μm thick on the plasma-nitrided surface, and the effective nitriding layer depth reached 0.4~0.5 mm, which increased the surface hardness of the substrate by more than 93%. The wear mechanisms of plasma-nitrided surface under dry conditions were mainly abrasive wear, delamination, and oxidation wear. The micron-scale spherical ferric nitride particles formed by plasma sputtering easily peel off under the action of tangential cyclic stress. They were collected by surface texture to form a third-body abrasive bed, which played a solid lubrication role in the contact interface. Especially in the large-displacement fretting slip region (D=60 μm), the friction coefficient and dissipation energy of the plasma-nitrided surface were reduced by 18.5% and 10% compared with those of the substrate, which showed an excellent anti-friction effect. Furthermore, the delamination wear on the plasma-nitrided surface disappeared under grease lubrication condition, and slight abrasive wear and oxidation wear were the main wear mechanisms. Notably, the frictional dissipation energy and wear rate of the plasma-nitrided surface were reduced by about 28.6% and 60% compared with the substrate due to the synergistic lubrication effect formed by the interaction between the grease and third-bodies on the fretting interface. The abovementioned results indicated that the performance of plasma-nitrided surface to alleviate fretting damage was significantly improved under the same service conditions and vibration energy input. This research provided theoretical data and application references for anti-fretting surface engineering protection of railway vehicle key components.