A total of 17 ports － the most compared with other mainland cities － link Shenzhen with the outside world. Nine commercial ports are sprawled 15 kilometers along the city's coastline, from where more than 40 world shipping companies sail on more than 150 container transport routes. The turnover volume of containers in the Shenzhen ports is fourth in the world. The number of passengers using Shenzhen International Airport is fourth among Chinese cities. The Beijing-Guangzhou Railway and the Beijing-Kowloon Railway converge in Shenzhen, making it an important hub of railroad transport. The city's first Metro line was opened in 2004.