The City Line doubles main line rail capacity through the centre of the city, diverting suburban trains away from the congested double-track line between Stockholm Central and Stockholm South.
The project included the construction of new underground platforms at Stockholm Central and a new station at Odenplan, which replaced the existing surface platforms at Karlberg.
A full description of the project can be viewed here.
https://www.railjournal.com/index.php/commuter-rail/taking-the-sting-out-of-the-wasps-waist.html