Mention of "Burma" and "Railway" in the same sentence inevitably calls to mind the famous, or rather infamous, Thailand - Burma Railway, constructed by Japanese forces during World War II using the labour of Allied prisoners of war, many of whom lost their lives.
However, when this line was constructed, Burma / Myanmar already had an extensive network of metre gauge railway lines, and it is these lines that constitute the present network. The Thailand - Burma Railway fell into disuse at the end of the war and the section in Burma (which connected with the existing network at Thanbyuzayat, between Mawlamyine (Mouleim) and Ye) was never reopened.