The plans for what would be the world’s biggest and most challenging construction project, official Chinese media reported Thursday, foresee a high speed rail-link starting in north-east China, then heading north-east to the tip of Siberia, nearest Alaska.From there, a 200 kilometer (125 mile) tunnel would link Asia with North America for the first time as the railway headed under the Bering Strait into Alaska, through Canada then south towards US West Coast cities.The project is one of a number of ambitious high-speed international rail-links at various stages of development intended to link China with the rest of the world. But the proposed China-US link faces a number of novel civil engineering challenges to overcome if it is ever to come to fruition.According to Wang Mengshu, a railway expert at the Chinese Academy of Engineering, quoted in the Beijing Times , the project, known as the “Russia plus the United States high-speed line," would run for 13,000 kilometers (8125 miles). Trains, possibly using the magnetic levitation technology developed and already in use on China’s Shanghai Maglev train , would travel at an average of 350 kph (220 mph). The entire journey from China to the US would take two days."Right now we're already in discussions. Russia has already been thinking about this for many years," The Guardian reports Wang saying.The Beijing-US rail link is just one of four major international high-speed rail lines that China hopes to construct. One proposed Eurasian transcontinental line would run from London, via Paris, Berlin, Warsaw, Kiev and Moscow before splitting in two. One branch would become a modern Trans-Siberian railway equivalent and the other would head towards China through Kazakhstan.A second line starts at Urumqi in China’s far west then heads to Germany, taking in Kazakhstan, Uzbekistan, Turkmenistan, Iran and Turkey en route.The third major rail project runs from the south-western Chinese city of Kunming , terminating at Singapore.The obvious civil engineering hurdle that any Beijing-US railway would have to overcome is the physical barrier the Bering Strait presents. The proposed tunnel, 125 miles long, would be four times longer than the Channel Tunnel that connects the UK and France. But tunnel technology has moved on in the 20 years since the Channel Tunnel was completed. China is already considering another futuristic tunnel project, the Bohai Strait Tunnel linking north-east China with the Shandong Peninsula. The Bohai Strait Tunnel would be 123 kilometers (77 miles) long.The less obvious civil engineering challenge concerns the terrain. Much of any proposed new rail-line would pass through tundra regions of Siberia, Alaska and northern Canada. Climate change is already affecting the tundra and some areas of stable permafrost, which would otherwise have lent solidity to any railway above, are becoming unstable and "spongy" as global warming takes hold.Such sponginess is anathema to maglev technology, as engineers found to their cost when constructing the Shanghai Maglev line. There, the alluvial soils meant that supporting columns, necessary to ensure the maglev guideway met stability and precision criteria, had to be constructed every 25 meters instead of the projected 50 meters apart.Another obstacle may be the cost. Again, taking the Shanghai Maglev line for the purpose of back-of-envelope calculations, the Shanghai route — just over 30 kilometers long — cost $1.2 billion at 2003 prices. Without allowing for inflation in the intervening years, that would put the cost of the Beijing-US railway line at somewhere north of $511 billion.It looks like the refrain, “The train standing at platform 1, destination Beijing...” might not be echoing around Union Station, L.A., at least not for a few years.