The most famous portion of the railway is Bridge 277, "the bridge on the River Kwai", which was built over a stretch of river which was then known as part of the Mae Klong. The greater part of the Thai part of the route followed the valley of the Khwae Noi River (Khwae: branch or tributary; Noi: small; Khwae is frequently mispronounced by non-Thai speakers as "Kwai", the Thai word for water buffalo). This gave rise to the name "River Kwai" in English. In 1960, because of the discrepancy between fact and fiction, the part of the Mae Klong which passes under the famous bridge was renamed as the Khwae Yai (Thai แควใหญ่, English "big tributary").
This bridge was immortalised by Pierre Boulle in his book and the film based on it, The Bridge on the River Kwai. However, there are many who point out that both Boulle's story and the film based on it were utterly unrealistic and do not show how bad the conditions and treatment of prisoners were.[18] On the part of the Japanese, many resented the movie's depiction that their engineers' capabilities were inferior to those of British engineers. In fact, Japanese engineers had been surveying the route of the railway since 1937 and they were highly organized.[19] The Japanese also accused the film's "glorification of the superiority of Western civilization" because the British in the film were able to build a bridge that the Japanese could not.[20]
Boulle outlined the reasoning which led him to conceive the character of Lt-Col Nicholson who works to build the fictional bridge and ultimately tries to prevent its destruction in an interview which forms part of the 1969 BBC2 documentary "Return to the River Kwai" made by former POW John Coast. A transcript of the interview and the documentary as a whole can be found in the new edition of John Coast's book "Railroad of Death".[16] The first wooden bridge over the Khwae Yai was finished in February 1943, followed by a concrete and steel bridge in June 1943. It was this bridge 277 that was meant to be attacked with the use of the first-ever example of a precision-guided munition in American service, the VB-1 Azon MCLOS-guided 1,000 lb ordnance on 23 January 1945[21] but bad weather scrubbed the mission.
According to Hellfire Tours in Thailand, "The two bridges were successfully bombed on 13 February 1945 by the Royal Air Force. Repairs were carried out by POW labour and by April the wooden trestle bridge was back in operation. On 3 April a second raid by Liberator bombers of the U.S. Army Air Forces damaged the wooden bridge once again. Repair work continued and both bridges were operational again by the end of May. A second raid by the R.A.F. on 24 June put the railway out of commission for the rest of the war. After the Japanese surrender, the British Army removed 3.9 kilometers of track on the Thai-Burma border. A survey of the track had shown that its poor construction would not support commercial traffic. The track was sold to Thai Railways and the 130 km Ban Pong–Namtok section relaid and is in use today."[22]
The new railway did not fully connect with the Burmese system, as no bridge crossed the river between Moulmein on the south bank with Martaban on the north bank. Thus ferries were needed. A bridge was not built until Thanlwin Bridge in 2000–05.