BEIJING -- By focusing on passenger needs, and efficient operations, China's high-speed rail (HSR) traffic is expected to continue rapid growth over the coming two decades, said a World Bank paper.

China has the world's largest HSR network, but passenger numbers have been the subject of debate.

A World Bank paper released Friday finds initial traffic volumes are promising, with traffic growing from 128 million trips in 2008 to 672 million trips in 2013. In 2013, China's high-speed rail lines carried more passenger-kilometers (214 billion) than the rest of the world combined, about 2.5 times the volume in Japan and four times the volume in France.

China is a very large country with a high population density, widely spaced large cities, and economic rebalancing strategies go for the long-term success of HSR.

A survey by the World Bank, China Railway Corporation and the Third Railway Survey and Design Institute indicates that a large proportion of high-speed train passengers are between the ages of 25 and 55, with many using the HSR for business travel.

The survey shows the average income of high-speed train passengers was 35 to 50 percent higher than that of conventional train passengers.

"Understanding and addressing passenger needs are critical to achieving the full impact of the HSR network. While initial results are encouraging, HSR remains a major investment that requires high traffic density to be justified economically and financially," said Gerald Ollivier, a World Bank senior transport specialist and co-author of the paper.

"This can be achieved by working closely with cities to develop areas around stations in a way that leverages the gain in accessibility that HSR provides," Ollivier said.

It is important to optimize train frequencies and city pairing, introduce flexible ticket prices reflecting peak and off-peak periods, and introduce convenient e-ticketing services. "By focusing on these aspects, and on the efficient and effective operation of the network, HSR in China can continue to experience substantial growth for many years to come," he added.

The World Bank has provided loans to support six railway projects in China.