The sealing function in the braking system of the French high-speed train (TGV) is provided by elastomeric components, such as nitrile butadiene rubber (NBR) O-rings. Like most elastomers, these seals are sensitive to thermo-oxidative aging. In the current application, aging results in a strong hardening of the O-rings that alters the sealing function and impacts the lifetime of the mechanical braking system. This places severe demands on maintenance operations. Therefore, understanding the origin of this aging and studying its effects on the material properties is of significant importance to limit train maintenance operations.In the present study, accelerated aging tests have been carried out with different NBR formulations at different temperatures. These tests allowed to reproduce ex-situ the hardening observed on the O-rings. Aging mechanisms were identified and related to the aging conditions of the NBR formulations through different characterization techniques: infrared spectroscopy, swelling tests, X-ray fluorescence spectrometry, differential scanning calorimetry, thermogravimetric analysis and micro-hardness.