China's state-owned rail corporation is more than $600 billion in debt, reports said, almost twice the size of Greece's obligations.

The China Railway Corporation (CRC) operates the country's trains, including an already world-beating 19,000-kilometre (11,800-mile) high-speed rail network, with at least another 11,000 kilometres planned.

But according to a recently released financial report, it owed 4.14 trillion yuan ($614 billion) at the end of April, said respected financial portal Caixin.

In comparison, Greece, whose debt crisis has threatened the eurozone and needed repeated bailouts, had an estimated public debt of 311 billion euros ($356 billion) at the end of last year, according to the European Union's Eurostat.

CRC's borrowing increased by over eight percent year-on-year, the numbers showed, a rise driven by the country's fever for expanding the network of super-fast trains, a point of national pride.

But China has seen a decline in rail freight, a major source of CRC's revenue, the Global Times newspaper reported Thursday.

The debt number "keeps growing", Zhao Jian of Beijing Jiatong University told the paper, adding: "This business model isn't sustainable."

Company losses rose 35 percent year-on-year to 8.73 billion yuan in the first quarter, the paper reported.

China is struggling to move its economy away from its dependence on massive construction projects and exports as the main drivers of growth.

But the country's addiction to massive infrastructure injections to fuel GDP expansion has proven hard to shake.

Rail, in particular, has continued to absorb huge amounts of capital as Beijing continues to push its ultra-modern train system into sparsely populated western regions.

China's economy, a vital driver of global expansion, grew 6.9 percent last year, its weakest rate in a quarter of a century.