The Japanese used the local Indonesians, mainly those from Java, as slave labour on the railway, and other projects around South-East Asia. Even before the Dutch capitulation in 1942, the Japanese had identified Java as a major source of labour, and in the beginning promised good food, wages and accommodation to carry out easy work for the Japanese. When the number of volunteers for these jobs declined as word got out about the conditions that the romushas were being forced to work under; the Japanese began to use force and intimidation under the threat of punishment to 'collect' a further hundred thousand new labourers.
On the Pakan Baroe railway, the romushas were used to build the embankments, and to build camps which the following allied prisoners would move into. The treatment received by these romushas was much worse than that of the allied prisoners. They had little food, and no medical treatment which led to the sick being left to die alongside the train line.
The Japanese liked to use dynamite to blow up sections of mountainside and hills so the track could be built through them, rather than building tunnels, as the Dutch surveyors had suggested. When this occurred, they would not give warning to the romushas working in the area, and it was common for them to bring down large amounts of rubble on the romushas. The following work parties then had to clean away both the blown rubble and romushas bodies.
There are varying reports on the number of romushas who were sent to work on the railway, from around 15,000 to 120,000. However, most resources I have found indicate that the higher number is correct, and so this is the one I have always worked with. Out of the 120,000 Javanese slaves sent to Sumatra to work on the railway (they were also sent to camps on other parts of the island, such as Medan and Bukitinggi); an estimated 23,000 are thought to have survived their 2 1/2 years working on the railway. This is what all the estimates of romusha numbers agree on: there was around a 20% survival rate. This is compared to the estimated 85% survival rate of the allied prisoners.
When liberation of the allied prisoners and romushas occurred, little was done to ensure the well being of the romushas after the allies left. This led to the majority of all Javanese romushas never returning home, and staying on Sumatra around the railway location, where some still remain to this day.