The Burmese portion of the TBR was under the command responsibility of the Railway #5 HQ group. In many ways this area mirrored that on the longer Thai side. In each portion of the TBR the first 50 or so Kms ran over flat open terrain over which rail were laid fairly easily and quickly.
The mountainous area that forms the Thai-Burma border presented many of the same problems for the engineers on both sides. The primary difference being that there was a more gradual rise into the mountains on the Burmese side that did not require the switch-backs as in the Hintok area. Logistics on both sides of the border were always an issue; except that Burmese-side relied solely on overland transport of goods while the Thai-side had the luxury of the river.
All of the Burmese rivers ran north to south across the path of the railway. This required the building of four of the largest bridges on the TBR in Burma. The US POWs participated in the construction of some of these.
The Fitzsimmons Party spent almost its entire TBR time in the ‘lower’ area nearer Thanbyuzayat in the 18 to 25 Kilo area. The main body of US POWs worked in the highlands area in the 80 – 110 area and were later sent to a wood cutting camp just inside the Thai border before all were consolidated to the Kanchanaburi area camps.
A Commonwealth War Graves Commission cemetery and museum are also maintained at Thanbyuzayat but due to its remoteness and Burmese Visa requirements, it is rarely visited.
This YOUTUBE video gives brief glimpse of the privately-run museum:
Unfortunately, none of the on-line references that I have found even acknowledge the presence of the US POWs in this portion of the TBR.
This gallery of photos are mainly from the on-line posts of the Australian War Museum but do not identify the exact site.