HONG KONG — After months of debate and political discord, passengers started boarding high-speed trains at a new station in Hong Kong on Sunday morning, the formal launch of a multibillion-dollar transportation link that will tie the former British colony more closely to the rest of China.

Another project, the world’s longest sea bridge, is expected to open later this year. Like the train station, it is both an impressive engineering feat and a source of controversy. It will span the mouth of the Pearl River, linking Hong Kong with the mainland city of Zhuhai and the former Portuguese colony of Macau, the world’s biggest gambling hub.

Hong Kong officials say the projects are critical to economic development and will speed the movement of goods and people through the region, which the Chinese government wants to bind more tightly together. But many residents are concerned about what a Greater Bay Area, as China calls its vision of a more closely knit Pearl River Delta region, will mean for the city’s unique identity.