This study investigates a flexible marshalling mode to address the pronounced temporal-spatial imbalance of passenger flows in urban rail transit, achieve a more precise matching between supply (capacity) and demand (ridership), and thereby reduce operating costs. A comprehensive analysis is conducted on the current state of research on flexible marshalling, its passenger-flow adaptability, and engineering implementation requirements; using Qingdao Metro Line 5 as a case, the study further structures the online coupling/decoupling workflow and examines train operation routing design, fleet allocation strategy, track layout requirements, capacity evaluation, and life-cycle operational economic benefits. Results show that online coupling/decoupling is best located within the track arrangement zones of turnback terminals and junction stations, and that configuring terminals with paired turnback sidings increases turnback capacity. Additional capital expenditures attributable to the flexible mechanical marshalling mode are dominated by rolling stock and signalling systems, which together account for approximately 90%; it is therefore recommended that civil works reserve the necessary interfaces (e.g., for track layout adjustments and platform screen doors) during initial construction. Compared with an all-6-car fixed formation baseline, a mixed operation pattern comprising conventional 6-car fixed formations and 3+3 flexible coupled sets yields superior cost effectiveness through notable reductions in traction energy consumption and maintenance costs; nevertheless, current deployment remains constrained by limited system capacity, higher marginal equipment investment, and increased operational management complexity, all requiring further optimization. The findings provide a technical and organizational reference for designing train operation schemes on comparable urban rail lines.