In the Second World War, after the occupation of Thailand and Birma, the Japanese army had many difficulties in providing its troops with supplies. The delivery of supplies by sea through the Strait of Malacca was difficult because of air raids by the allied armies. Therefore the Japanese government decided to construct a railway between Thailand and Birma. Allied prisoners of war and Asian coolies were forced to work on the railway project. In various places primitive camps were established for the labourers. They had to construct the railway under inhuman circumstances. The construction works started at 16 september 1942. The Japanese had planned to finish the project in five years, but already after fifteen months the railway was ready for use. Many people died in the construction of the railway, most of them because of hunger, exhaustion, tropical disease and air raids. For 20 months the Japanese military made use of the railway until it was destroyed by allied bombardments. After the war the British dismantled a large part of the railway near the Birmese border. The rest was sold to the Thai railway company.
The original bridge over the river Kwai was made of bamboo. The bridge was temporary and in the course of the war it was replaced by a steel bridge. For this project the Japanese army dismantled a steel bridge in Java and transported it in parts to Kanchanaburi where allied prisoners of war reassembled the bridge. The original steel bridge had semicircular arches, but in 1945, at the end of the war, three arches (the fourth, fifth and sixth in the middle) were destroyed by British bombers from Ceylon. The Thai railway company replaced them by two rectangular steel arches with a curious inscription "Made in Japan". The rectangular arches came from a steel factory in Tokio and were meant as recovery payments for the war damages. Near the bridge is a railway museum with original parts of the railway and a locomotive which was used at the time of the construction for the transport of men and materials. A walk over the bridge is hazardous because of loose girders and shelfs with large slits showing the river below. Some times each day a train slowly and carefully crosses the bridge. On its way to Kanachanaburi the train passes the Wang Po viaduct, a high wooden bridge built against a giant rock wall and for a large part still consisting of the original construction materials.
The Sai Yok National Park is in the environment of Kanchanaburi. The park has various waterfalls in a forest landscape with bamboo and teak trees. The waterfalls are concentrated on five altitudes. Each waterfall ends up in a little pond fit for recreation.