This view shows excavation for a line change.
Steam shovel is on temporary tracks. As material is removed from front of shovel more temporary rail would be placed in order for shovel to move forward.
The third, outside rail, is, of course, for the dummy and cars.
At some point the outside standard gauge rail would provide access to two rail track for the dummy. This would be in order for the dummy to clear regular through trains.
As the shovel advances, so too would the "narrow gauge" rail.
After completion of excavation, grading, etc. main track would be relocated.
Jimmy