This paper focuses on the energy consumption of urban rail transit trains with flexible train compositions. An energy-efficient speed profile model is proposed to calculate the motion state of various train types. Considering the traction energy consumption and regenerative braking energy, an energy consumption model is established to assess the total energy consumption under a given flexible train service plan and timetable. A set of numerical experiments based on real metro line data is conducted to analyze the energy consumption of different train operation scenarios. The results show that the energy consumption of the flexible train composition mode is reduced by 11.70% compared to the fixed train composition mode, highlighting its potential for enhancing energy efficiency and sustainability in urban rail transit systems.