Originally the IJA plan called for the entire railway to be built by ‘hired’ indigenous Asian workers. But after the fall of Singapore and the capitulation of the Dutch in the East Indies, they found themselves with nearly 200,000 Allied military POWs that they had not expected. They decided to use them alongside the Asian workers to construct the railway in the shortest time possible.
In May 1942, the first 3000 Allied POWs (mostly Australians) known as A-Force arrived in Burma. By June, they and ‘hired’ Burmese and ethnic Mon laborers had begun laying rails south from Thanbyuzayat. Also in mid-June, 3000 mostly British POWs known as Group 1 arrived at the Nong PlaDuk camp in Thailand. According to the official marker erected by the Japanese there, the start date of the Thai portion of the railway was set as 16 Sep.
Thai workers were hired to build the first 50 kilometers to Kanchanaburi. Since that portion, like in Burma, was over flat, unobstructed, agricultural land, this first section was completed in just a matter of weeks. In November, about 1500 British POWs were moved from Nong PlaDuk to ThaMaKam where they began to build the first of two bridges over the Mea Klong River. Just prior to that, the Headquarters of the IJA engineering and support staff (perhaps as many as 5000 soldiers at its peak) was also moved from BanPong to Kanchanaburi.
The TBR was built in sections and two different IJA Railway Units[1] were tasked with the job. One worked east from Thanbyuzayat Burma and the other west from NongPlaDuc Thailand. The first 50 or so kilometers in each direction were built by locally hired workers: Burmese or Thais. Since it was early in the process and they were working near their homes, they had an entirely different experience than did the later arriving romusha.
As trains began arriving from the south, a transit camp was established at Wat Don Toom in BanPong, Ratchburi. There the trains would halt after the multi-day journey and the occupants (either POWs or AFL) would be pushed off the cars and into the temple grounds. From there, they would begin the trek to their assigned places on the TBR. Most walked over 100 Kms (minimum) to those camps. Again, unlike the Allied POWs being brought from Singapore, they had no organization, no leadership and no real ‘kit’, except what they were carrying.
Once again, it must be noted that we have no records what so ever pertaining the timing and numbers of romusha who passed through this camp. We know only that the railway was the only means of transport of all of the workers and POWs from the south.
Records indicate that the Allied POWs were almost exclusively responsible for building the TBR from Kanchanaburi to the WangPo trestle. It was at camps beyond Kilo 120 that the vast majority of the romusha were forced to work. In some cases, it is documented that they worked alongside Allied POWs (at HellFire Pass for example). But in other cases, they worked separately (at the Pack of Cards trestle at Hintok).
[1] The 5th Regiment in Burma and the 9th in Thailand