Quick history reminder:
The 415km of the death railway connecting Thailand to Burma were built between 1942 and 1943 by the prisoners of war (mainly Australian, British, American and Dutch) and south east asian workers under Japanese order.
The conditions were terrible and during some period death rate was above 40%. A lot of the railway was bombed by the allies in 1945 with many bridges destroyed, a significant length is also now under water at a dam near the 3 pagodas pass. The only part still in use is between Nam Tock and Bangkok, passing Kantchanaburi and the famous river Kwai bridge.
Note that the river was re named after the film as originally the death railway followed the river Kwai for kilometers but never crossed it, but it did cross several times the river Mae Kwong which was then renamed big Kwai. (isn't Asia amazing to sort these little details out? No problem, just rename the river! As long as the tourists come...)
Back to our visit. At the memorial, we walked some part of the railway up to the hellfire pass, the longest cutting made in the rock in the railway.
Final note, the western pronunciation of Kwai means water buffalo, the correct pronunciation is "ware" like in Tupperware!