China only opened its high-speed rail service to passengers four years ago, but it now boasts the world\'s biggest network, with more than 8,300 kilometres of track -- a figure that is growing rapidly. Trains are by far the most popular form of long-distance transport in the country of more than 1.3 billion people where, despite three decades of blistering economic growth, air travel remains far too expensive for most people. China recorded more than 1.68 billion passenger trips on its network in 2010, dwarfing the 248 million airline passenger journeys in the same period. The vast country\'s 91,000-kilometre (57,000-mile) rail network carries hundreds of millions of people every year and stretches from the eastern metropolis of Shanghai to the remote Tibetan plateau. In recent years, the government has invested heavily in high-speed train links as it strives to meet the growing demands on the transport system. Last year, government spending on the high-speed railway surpassed 700 billion yuan ($109 billion), and China is now home to around half the world\'s high-speed tracks. The network officially opened to passengers in 2007 with a link between China\'s commercial centre, Shanghai, and the city of Suzhou, 112 kilometres to the west. By the end of last year it covered 8,358 kilometres, and it is expected to exceed 13,000 kilometres by 2012 and 16,000 kilometres by 2020. But even before a collision Saturday between two high-speed trains triggered what the government called an \"urgent\" overhaul, there were fears that corruption and the breakneck pace of development could have compromised safety. China\'s state auditor said construction companies and individuals last year siphoned off 187 million yuan from the Beijing-Shanghai project, which has been plagued by power cuts since opening to great fanfare last month. Beijing sacked railways minister Liu Zhijun in February for allegedly taking more than 800 million yuan in kickbacks linked to contracts for high-speed rail expansion. There are also growing complaints that ticket prices are too high for the low-paid masses. The $33 billion high-speed track linking Beijing and Shanghai opened to passengers on June 30 -- a year ahead of schedule and a day before celebrations to mark the 90th birthday of China\'s Communist party. But sales of seats on the flagship link have been slow. Ren Xianfang, a senior analyst at IHS Global Insight in Beijing, said the pace of development in China had been \"phenomenal\", but warned it could not be sustained. \"This (accident) could be a setback to the high-speed train development,\" she told AFP. \"I now feel nervous about taking passenger trains because it has confirmed our worries about potential safety issues with the high-speed trains. I think this is a very bad situation for the government.\" Foreign companies participating in the development of high-speed rail in China have also accused Chinese state-owned companies of stealing some of their technology or forcing them to transfer their know-how -- charges China denies. China\'s high-speed trains were based on foreign technology but it had improved the technology to make the trains even faster, He Huawu, chief engineer of the railways ministry, said last July. Authorities have reduced the maximum speed of bullet trains to 300 km per hour to make journeys safer and more affordable.