A railway route between Burma and Thailand, crossing Three Pagodas Pass and following the valley of the Kwhae Noi river in Thailand had been surveyed by the British government of Burma as early as 1885, but the proposed course of the line – through hilly jungle terrain divided by many rivers – was considered too difficult to undertake.[4]
In early 1942, Japanese forces invaded Burma and seized control of the colony from the United Kingdom. To supply their forces in Burma, the Japanese depended upon the sea, bringing supplies and troops to Burma around the Malay peninsula and through the Strait of Malacca and the Andaman Sea. This route was vulnerable to attack by Allied submarines, especially after the Japanese defeat at the Battle of Midway in June 1942. To avoid a hazardous 2,000 miles (3,200 km) sea journey around the Malay peninsula, a railway from Bangkok, Thailand to Rangoon, Burma seemed a feasible alternative.[5] The Japanese began the project in June 1942.