To safely control a group of trains, it is essential to detect the position of train. The system to detect the position of train by the train itself will become a mainstream of the new systems that use radio transmission for a Computer and Radio Aided Train Control System (CARAT). This paper introduces the methods of train position detection that have been studied or tested, introduces the trend of these methods and reports the results of performance tests we have performed. It also describes the fundamental concept and approach to realize a system to detect the position of train which is applicable to CARAT and other systems.