The railway line L50A is one of the most important lines in Belgium, connecting the capital Brussels with Ghent. The line originally consisted out of two tracks. Because this formed a constricting bottleneck for the entire railway network in Belgium, the existing capacity was expanded. Between 2008 and 2015 a third and fourth track was constructed, one on each side of the original 2 tracks. Now that the expansion works have ended, the 2 original tracks have to be refurbished. The necessary works include: renewal of ballast and rails, changing all wooden sleepers by pretensioned concrete ones, strengthening of the subgrade and renewal and improvement of the waterproofing layers at the bridges and viaducts. This article focuses on the first 10 kilometers of the trajectory, including 10 locations where the waterproofing has to be renewed. At each location, the existing ballast, sleepers and tracks have to be removed for the middle two tracks, so a new polyethylene-waterproofing layer can be installed. Afterwards the tracks have to be restored. All of this has to be performed while the 2 outer tracks remain in operation and without disturbing the electrification cables above the tracks. Because of this, the project will necessitate a combination of creative planning, use of modified and small-scale equipment and a judicious use of night-time labor. All of these items will be discussed in the article.