Abstract: Based on the analysis of the structures of train diagrams, the features of V form skylights on busy trunk lines are studied, and a mathematical model is proposed to analyze the key factors in a V form skylight, which influence the carrying capacity of railways. Sensitivity analysis on the factors in a V form skylight shows that the deduction of carrying capacity of a railway line is directly proportional to the time spans and the lengths of V form skylights and inversely proportional to the speed of freight trains and the intervals between trains.