In metropolitan rail systems, passengers must board and alight expeditiously, rendering the precise control of door opening and closing times imperative for operational efficiency and safety. According to the European standard EN14752, the operational time of train doors must be adjustable within 3 to 5 s. However, the localization and system renewal plan necessitates upgrading the existing door control system without modifying the door mechanism or motor, which poses challenges to system stability and control accuracy. The low resolution of the motor encoder has been demonstrated to impede the efficacy of traditional current-based control methodologies, resulting in sluggish responses and significant challenges in accurately regulating the door’s motion to meet regulatory requirements. The development of an optimal method within the constraints of the existing hardware architecture to enhance the effective utilization of their sensing signals is challenging. In this study, we propose a variable-step speed control strategy, which is a simple yet efficient control method that does not require replacing existing sensing devices. The designed system utilizes the speed encoder to estimate real-time speed and implements pulse-width modulation (PWM) duty cycle adjustment for closed-loop speed control. The system dynamically adjusts the step size of the PWM duty cycle on the basis of the magnitude of the speed error. In the case of larger errors, a larger step size is employed to approach the target speed expeditiously. Concurrently, fine adjustments are implemented to ensure smooth operation and mitigate overshoot for minor errors. This method effectively overcomes the limitations caused by low encoder resolution, enhances the stability and reliability of door movement, and ensures compliance with safety regulations. Experimental results show that the door opens within 3.5 s, complying with safety regulations. This indicates that the proposed method facilitates the opening and closing of metro train doors within 5 s, thereby adhering to the established EN14752 standard. This approach offers a cost-effective and reliable solution for metro train door control.