Abstract: Based on the analysis of the structure of train diagrams, the conditions for specific overtaking modes to take place are discussed. The carrying capacity and the speed of passenger trains are analyzed on condition that express trains are to be put intooperation on the existing double track railways and the number of tracks in intermediate stations to be unchanged. A probability model for layout of overtaking stations for express trains is proposed. Simulation is conducted by taking the Beijing-Shanghai and Beijing-Guangzhou railways for example. Some principle for layout of overtaking stations for express trains along existing double track railways are suggested.