The Arlanda Express project included the expansion to a four track main line between Solna and Rosersberg, but the two extra tracks is indeed not dedicated to Arlanda Express only.
But this makes for quite a good capacity on the northbound line and, as we know, Arlanda Express typically is on time. The problem is the southbound main line (which Arlanda Express doesn´t use).
And the problems last week was due to the construction work taking place for the new Citybanan, which aims to increase capacity on the southbound main line.