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.