The subway system in China's capital city is one of the world's busiest public transit systems, carrying millions of passengers each day. It is also likely one of the biggest losers among them, running at a loss large enough to build another iconic Bird's Nest stadium of the kind originally built for the 2008 Olympics every year.

Despite a large passenger turnover— during rush hour, people wait for several trains to just get on one -- the construction and maintenance costs, coupled with the cheap, flat price of tickets, which cost just two yuan(32 cents), seem to be too heavy a burden for the system.

Last year, a total of 1.74 billion passengers took the subway in Beijing, according to the Beijing Municipal Commission of Reform and Development. Many of Beijing's 21 million residents prefer the subway as a way to avoid traffic jams on the ground.

Still, the operator suffered a loss of 3.46 billion yuan ($558 million) last year, official data showed, with operating expenditures of 6.68 billion yuan and operating revenue of 3.22 billion yuan. That loss for the Beijing Mass Transit Railway Operation Corp., Ltd is more than what it cost to build the Olympic Bird Nest stadium, now a major landmark in Beijing, which cost 3.13 billion yuan to build.

The annual costs to run the subway nearly quadrupled from 1.34 billion yuan in 2007 to 5.33 billion yuan in 2013, according to a Xinhua News Agency report carried on the central government website last week.

The reasons? In addition to operational costs, the city has been building up its subway lines at a frenetic pace, and each kilometer of new subway line costs about 1 billion yuan to build, the Beijing Evening News reported on Monday. Beijing is also planning to spend another 400 billion yuan on such efforts, building up a subway system of more than 1,000 kilometers by 2020, up from the current 465 kilometers, the newspaper said.

At that point, the paper noted, in order to break even, the cost of a ticket would need to be tripled to around 6 yuan per passenger.

In the meantime, to keep the system running, Beijing's government has supported the operator for the past seven years with 22.1 billion yuan in subsidies, Xinhua said.

Beijing has been wanting to increase subway ticket prices, which haven't been raised for seven years. In December, the government first floated the idea of ticket prices, and in March confirmed plans to shift to a distance-based pricing plan. The news picked up more steam again in May when a photograph of a document supposedly outlining the specific price increases began circulating on social media. The document said that the fare for trips of less than 6 kilometers would rise to 3 yuan, while trips over 16 kilometers would be capped at 5 yuan.

Public outcry to the suggestions has been widespread. Still, subway operators in other Chinese cities – such as Shanghai – already use distance-based pricing plans. In Shanghai, it could easily cost at least 10 yuan per ride to get around the sprawling city, and such higher costs have benefited the local subway operator. The Shanghai Shentong Metro Co., Ltd. said in its annual report that the net profits attributable to its shareholders rose 8.62% in 2013, to 121 million yuan.

And while no one likes to pay higher prices, some residents of the Chinese capital – which boasts a population nearly the size of Australia –- say that fare hikes are necessary.

"A cheap public transit system will eventually crush Beijing," wrote one Weibo user earlier this month. "There's a large number of migrant population that's pouring into Beijing. The system will collapse someday."

-- Liyan Qi