Japan put itself under pressure as it widened its hold on the Far East, as its forces had to be supplied. Burma had a natural supply line for its troops, the Irrawaddy River which runs the full length of Burma and boats could therefore supply the Japanese forces pushing towards India, but this was slow. The Burmese railway also ran the full length of Burma, the only problem being there was no line between Malaya and Burma, this had to be bridged quickly. If this was joined to Malaya the Japanese forces could be supplied quickly and could carry on with the invasion of India.The route ran on the east bank of the Mae Khlong River from Bangkok until it reached  the Khwae Noi River, the track was then to cross the Mae Khlong and hug the east bank of the Khwae Noi until it reached the mountains in the north and cross the mountains at Three Pagodas Pass. It would then snake out of the mountains towards Thanbyuzayat. With this plan the river was a great advantage as it could help supply materials and the labour force needed to build the railway.
In peace time, plans to build a railway from Bangkok to Burma had been shelved because of the cost involved. Now, with over 100,000 prisoners taken in its advance, Japan had a workforce, to do with as it pleased.
A railway could now be built to help supply its forces on the Burma front and its advance into India for little cost to themselves, this was to prove a huge deficit in prisoners lives.
On 8 August 1942, the Prime Minister signed an agreement with the Japanese representative General Sheji Poriya to build the railway. The Japanese hoped that the single meter gauge railway would be able to transport 3000 tons of supplies and strategic materials a day.
The Death Railway branched off from the existing southern railway and headed towards Kanchanaburi. The first fifty-five km from Nong Pladuk to Kanchanaburi were easy to construct because of the flat terrain. The rest of the way was hell though and that is how it earned its nickname.
When the first survey on the railway was completed it was estimated that it would take five years to build. The principal engineer was S.O. No. Construction began in October of 1942 and it was finished in August of 1943. The railway was put into use on 25 October 1943. With two tracks, one starting from Thanbyuzayat in Burma and the other from Nong Pladuk met at Nieke just south of Three Pagodas Pass.