1. National Engineering Research Centre of Rail Transportation Operation and Control Systems, Beijing Jiaotong Univ., Beijing 100044, China (corresponding author).
2. State Key Laboratory of Rail Traffic Control and Safety, Beijing Jiaotong Univ., Beijing 100044, China.
3. Dept. of Computer Science, Univ. of York, Deramore Lane, Heslington, York YO10 5GH, UK.