In China, although per capita energy consumption is lower in the urban rail transit system than other modes of transportation, the total energy consumption and greenhouse gas emissions will reach considerable levels based on the current speed of urban rail transit system development. The objective of this research is to use a process-based life cycle assessment to define the system boundaries of the construction, operation, and maintenance processes of Shanghai Metro and to inventory the associated resource inputs (materials, fuels, and equipment) and emission outputs based on actual observed data. A comparative analysis of traction and station operation emissions and the infrastructure emissions structures of different urban rail transit systems around the world is also provided. The results show that the traction emissions per passenger-km traveled of Shanghai Metro are competitive at the global level, but there is still great energy-saving potential in the operation phase, especially in ornately designed train stations. The preliminary conclusions of this study may help shed light on the emission reduction potential of urban rail transit systems and the emission reduction targets in China and serve as a source of information and data for future LCAs.