This is a subway-style map that represents how to travel between destinations rather than portraying geographical accuracy. Train routes and borders have been straightened, and stations have been realigned to fit the style of the map. Most of the lines are current or have been proposed at some point. Some "Nomadic Notes Recommendations" have been added to fill in missing gaps.
This is not an actual map of Southeast Asia railways, so no responsibility will be held if you turn up to Nakhon Ratchasima waiting to catch the Angkorat to Siem Reap, because there is no such train, and Siem Reap doesn't have a railway (yet).
Details of the proposed railways can be found at: nomadicnotes.com