If one looks at a satellite view of that Kanchanaburi area such as seen in the GOOGLE MAPS software, it is obvious that the rails run about 5 Km NW of the old walled city of Kanchanaburi to the point where the bridges were built. Once across the river, the tracks essentially double-back on themselves over that same 5 KM length. The obvious question is: why was the railway lengthen by some 10 Kms? The answer is found in a book written by one of the primary engineers who surveyed and directed the building of the TBR: Across the Three Pagodas Pass: The Story of the Thai-Burma Railway by Yoshihiko Futamatsu (as translated by British ex-POW Ewart Escritt).
He describes how there were three possible routes considered initially. One would have started much farther north in central Thailand in the province of Pitsanulok. This route had been previously explored by German engineers in the late 1800s but was rejected due mainly to the gradient that the railway would have had to cross in terms of the elevation of the mountains that formed the border with Burma in that area. In a straight line distance it would have been a shorter hop to Moulmein, but also would have had to cross the Ping and Moei rivers; the latter being the Thai-Burma Border. It would also have had to use the existing Thai rails from Bangkok to Pitsanulok.
Once that choice was ruled out, the route from Nong Pladuk up to Kanchanaburi posed two alternatives. One would have taken the rails about 10 Km beyond Kanchanaburi city to Lat Ya where it would have turned to cross the Mae Klong (Kwae Yai) River and the turn NW towards the Three Pagodas Pass which would have been necessary to avoid the ruggedly mountainous areas that run north and south in that area. That alternate route would have met the route of the railway as it was built somewhere near NamTok (before Hellfire Pass).
Futamatsu relates that the initial party of Japanese Railway engineers led by a MAJ Hikiji used aerial maps to survey the possible routes and rejected those first two resulting in the one that followed the Kwae Noi River from ChungKai to the Three Pagodas Pass. It then became Futamatsu’s task to oversee that construction beginning at Nong Pladuk.
The reason why the railway took a 5 Km swing up and back is simple geology. The shortest route would have been to turn near the walled city of Kanchanaburi and cross the two rivers at about the point where they converge. But the engineers took core samples and determined that the river bed in that area was too silted to be able to easily reach the bedrock that would be needed for a stable bridge. So they went a bit farther north and found a more suitable site at Tha Mar Kam where the footing was more stable. They also seemed to listen to the locals who described how the Kwae Noi tributary — more so than its larger neighbor — flooded reliably at least once a year. So a bridge near its course would have been subjected to immense water pressure. The Tha Mar Kam site was a bit wider and the banks higher, so there was less of a flooding danger. I’m also sure that the inhabitants of the city were happy that the Allied bombs intended for those bridges were falling 5 Km north of their homes!
The follow series of maps published on the AUS 2/4 MG Bn site [https://2nd4thmgb.com.au/soldiers-all/camps-and-maps/ ]shows the terrain that determined the precise route the railway took (particularly maps 7 & 10):
8.1a Engineer Survey Teams
Here we have a rare photo of IJA Railway Engineers marking the path of the TBR. These survey teams were the true unsung heroes of this project if we might be allowed to credit their efforts. The decision of the general route for the Thai-Burma link was made purely on the basis of aerial surveys. It fell to these small teams of engineers to hack their way through virgin jungle to establish the exact path that the railway would take.
They would have had to subsist on the most meager of rations and supplies, mainly delivered by boat up the Kwae Noi River. But hunting and foraging must have consumed a considerable amount of their time and effort.
They would have had only the crudest of maps and aerial photos to suggest the path. They undoubtedly had to backtrack and redo sections of the survey as their path met unanticipated obstacles. Fortunately, the terrain in Thailand between Chung Kai and Nam Tok/Hintok was fairly flat and the route would have been easily mapped. They would have honed their survival skills over that 50-60 Km stretch. At Hintok, however, the task became infinitely more complex. They had to plan a route that would allow the locomotives to pull trains into the higher elevations of the route as it approached the Burmese border. Seemingly, they actually skipped this area and proceeded onto the plateau in the Sai Yok area to continue to lay out the right of way. It would fall to the bridge builders to span the gap with switchbacks and trestles as well as a handful of small cuttings to connect these upper and lower sections.
Unlike the way in which the berms and cuttings were made — simultaneously at many points – the survey had to be done linearly in a continuous line. IOW it was not possible to have multiple teams working simultaneously since it would be nearly impossible to connect up such independently surveyed sections.
These teams spent weeks alone in the jungles subsisting and surviving until a replacement team could locate and relieve them. Fortunately for those assigned to construct the berms and make the cuttings, the first 100 or so kilometers on both the Burmese and Thai Sectors were reasonably easy to survey so actual construction could proceed while the more difficult regions in the center half of the route was being slowly surveyed.