The £3.2 million operation between Craigendoran and Crianlarich started on Saturday November 9 and finished early on Wednesday (November 13) morning.
With news tracks, switches and crossings at Craigendoran, trains are now able to swap between lines.
The bridge at Garelochhead was also refurbished, with old paint blasted off by engineers and repairs to steelwork.
Replacement buses ensured those who use the train still had transportation over the few days the track was under construction.
Whilst the trains were not running, specialist geotechnical engineers had the opportunity to undertake earthworks inspections.
Ross Moran, Network Rail Scotland route director, said: “This vital work will help improve performance and reliability for our customers.
"Our engineers have worked around-the-clock to complete these projects quickly, with as little disruption as possible, and we thank our passengers and freight customers for their understanding while this crucial investment took place.”