A method, a device and a system for safely and reliably performing real-time speed measurement and continuous positioning are provided. With the method, inertial navigation data from an inertial navigation signal source arranged in a train is detected, and correction data from a correction signal source is detected. In a case that no correction data is detected, a current speed and a current position of the train is determined based on the inertial navigation data, and in a case that the correction data is detected, the inertial navigation data is corrected with the correction data, and a current speed and position of the train are determined based on the corrected inertial navigation data. Therefore, even in the case that no correction data is detected, the real-time speed measurement and continuous positioning can be performed safely and reliably based on the inertial navigation data.