Abstract: Objective Different from fixed train formations, the on-line flexible train formation in urban rail transit involves train decoupling/coupling operations. To fully account for the impact of such operations on train diagram compilation, it is necessary to conduct in-depth research on the key issues in train diagram compilation development under on-line flexible formation conditions. Method The process of train decoupling/coupling operations under different turn-back modes is analyzed. Suggestions are proposed for key train diagram compilation issues such as the selection of decoupling/coupling operation points, the calculation method for the number of decoupled and coupled trains, and the time required for train formation conversion. Result ＆ Conclusion Decoupling/coupling operation points should be set preferably at turn-back stations, especially those connected to depots. The number of trains to be decoupled and coupled depends on the operating interval of long-formation trains, train circulation time, and the location of decoupling/coupling operation points. Bidirectional decoupling/coupling require less formation conversion time compared to unidirectional operations. While the number of operation points does not affect conversion time in same-direction decoupling/coupling operations, it helps to alleviate the operational pressure.