HONG KONG — China began service Wednesday morning on the world’s longest high-speed rail line, covering a distance in eight hours that is about equal to that from New York to Key West, Fla., or from London across Europe to Belgrade.

Bullet trains traveling 186 miles per hour began regular service between Beijing and Guangzhou, the main metropolis in southeastern China. Older trains still in service on a parallel rail line take 21 hours; Amtrak trains from New York to Miami, a shorter distance, still take nearly 30 hours.