1. Department of Civil and Environmental Engineering, The Hong Kong Polytechnic University, Kowloon, Hong Kong, P. R. China
2. Hong Kong Branch of National Rail Transit Electrification and Automation Engineering Technology Research Center, The Hong Kong Polytechnic University, Kowloon, Hong Kong, P. R. China
3. School of Engineering, RMIT University, PO Box 71, Bundoora, VIC 3083, Australia