This study examines the key determinants influencing sustainability in urban metro rail systems, with a focused case analysis of the Delhi Metro. Rapid urban growth, rising travel demand, and environmental concerns have made sustainable mass transit essential for Indian cities. The Delhi Metro plays a major role in reducing traffic congestion, lowering air pollution, and supporting efficient urban mobility across the National Capital Region. This research evaluates how service quality, operational performance, connectivity, and environmental factors shape the overall sustainability of the metro system from the commuter's perspective, while also considering demographic diversity across age, gender, and residential locations. The study adopts a mixed-method approach using a structured questionnaire based on a five-point Likert scale, supported by secondary data from official reports. Primary data were collected from 421 metro users across NCR, and statistical analysis was conducted using SPSS through descriptive statistics, correlation, and regression techniques. The findings reveal that sustainability in the Delhi Metro is significantly influenced by multiple factors, with passenger comfort and safety emerging as the strongest determinant, followed by connectivity and traffic congestion reduction. Environmental factors such as lower energy use and reduced air pollution also contribute positively. The regression model confirms overall statistical significance, leading to the rejection of the null hypothesis. The study concludes that user-oriented service quality, network reach, and environmental performance are central to strengthening the long-term sustainability of urban metro rail systems like the Delhi Metro.