This is interesting - here's what the bumph says
During WW1 the allies needed to construct railways throughout France to help move men and supplies. The quickest method available was to construct a network of narrow gauge (60mm = 2 foot) train lines.
The film opens with scenes of Army personel laying the sleepers and track, ballasting work, loading and laying of pre-built trackwork. Next we take a ride on the train and see scenes of the railway in action as the little trains trundle along country lanes and through the town streets. The film ends with scenes of the train taking soldiers towards the front lines.