<p>Train Control Onboard Systems are typical systems capable of operating in degraded modes. Under normal conditions, they maintain operation at the highest functional level, while under specific exceptional circumstances, they can downgrade to a lower operational level to sustain service, thereby preventing train stoppages due to partial system failures. However, existing research on the resilience of Train Control Onboard Systems predominantly focuses on the impact of system performance degradation on resilience, often overlooking the influence of system degraded operation on both system performance and overall resilience. To address this gap, this paper proposes, for the first time, an Influence Index of EMU Operation Distance Deviation to quantify the impact of system degradation caused by sudden failures on train operation. The train operation under such scenarios is divided into three stages: degradation deceleration, degraded operation, and upgrade acceleration. Using availability as the resilience metric, a quantitative resilience model for the system under this scenario is established. Finally, taking a ground base station failure in the Chinese high-speed railway as a case study, a resilience assessment of the train control onboard system is conducted. The results indicate that degradation to CTCS-2 operation mitigates the expanding trend of the impact of the ground base station failure on train operation. Furthermore, it was found that the manifestation of the onboard system’s resilience exhibits hysteresis.</p>