The decision to use POW labor to build the TBR was actually made late in the planning stages. The British were the first to explore the concept of connecting Thailand and Burma — and in doing so forming a rail network that linked to Singapore and even Phnom Penh. They had mapped portions of two possible routes. One ran west from central Thailand starting at Pitsanulok, but the mountains along the border were a formidable obstacle.
The somewhat easier route that the Japanese finally adopted ran along the path of the Kwae Noi river from Kanchanaburi to the border crossing at Three Pagoda Pass. In Burma, it traversed a series of mountain ranges and crossed rivers in the valleys between them before flattening out as it approached within 50 Kms of Thanbyuzyat.
All of this area had been aerial mapped but it fell to a small group of intrepid Railway Engineer survey teams to walk and map the entire route.
The original concept was to hire tens of thousands of indigenous laborers, but in FEB-MAR 1942 the IJA suddenly found itself in possession of well over 100,000 Allied POWs mainly in Singapore and the Dutch East Indies. The majority of the US POWs were taken as POWs on the island of Java. They and their fellow Dutch and Australian POWs were then trans-shipped to Burma to work the TBR.
This might sound a bit crass when one considers the ordeal of the slave laborers, but I think that the unsung heroes of the TBR were the small survey teams that were sent into the raw jungle to literally map and mark each meter of the railway. There were no roads, no bridges, no way around the limestone out-croppings that were to be ‘cut through’.
These small teams of 5 -10 men spent months in the jungles living off the land; trekking out each day to map and record the route that the rails would take. We know almost nothing of them except a mention by Railway Engineer Futamasu who visited one such group early in their effort.
There is a group of young Thais who spend their weekends exploring the western regions of the TBR beyond Hellfire Pass where there is no longer a railway and where the jungle is reclaiming the railbed so laboriously constructed by the POWs and romusha. Their FACEBOOK page contains some videos of what these places look like today and a glimpse of what they must have looking like in 1942.
These are reportedly the documents in which the Thai gov’t transferred the usage of the land — a right of way of sorts — for the IJA to build the TBR from BanPong to Kanchanaburi: