The line capacity of a railway is affected by numerous factors, which varies under severe weather conditions due to equipment or facility impairments. To date, there is no general approach to capacity calculation. In this paper, the influencing factors on railway line capacity variations are identified under severe weather conditions. Based on the variations, the functional deterioration and fluctuation of facilities and equipment for train operations are investigated. The challenges of severe weather on train operations and a dynamic carrying capacity method for railway lines are explained. First, various states of the China Train Control System (CTCS) under severe ice and snow conditions are discussed. Second, descriptive and analysis models of train operation under a disaster situation are given based on a SPN (Stochastic Petri Network). Since the SPN and Markov Chain Model (MCM) are isomorphic, the transition probability of train operation conditions is analyzed in depth by MCM. Then dynamic line capacity transition probability and fuzzy comprehensive evaluation matrixes are constructed, and the Chapman-Kolmogorov (C-k) equation is adopted for analyzing the capacity variation of a railway line, as well as to provide a basis for decision making in operation plans during severe weather conditions.