Much of the railway network already exists, but there are still some major gaps. A big challenge is the differences in rail gauge across Eurasia. There are four different major rail gauges (to measure the distance between rails) across the continent: most of Europe, as well as Turkey, Iran, China, and the Koreas use the 1435 mm gauge, known as Standard gauge; Finland, Russia, and the former Soviet republics use a 1520 mm gauge; most of the railways in India, Pakistan, Bangladesh and Sri Lanka use a 1676 mm gauge, and most of Southeast Asia has metre-gauge. For the most part, the TAR would not change national gauges; large machines would be built to move shipping containers from train to train at the breaks of gauge.