Stockholm City Line

The Stockholm City Line is a commuter railway tunnel beneath central Stockholm in Sweden which is used by the Stockholm Commuter Rail. The line is 7.4 kilometres long, double track and electrified. It has two stations: Stockholm City Station is located directly below T-Centralen, the central station of the Stockholm Metro. The Odenplan station is the other station, and it is located next to the Odenplan metro station. The line entered service on 10 July 2017.
The tunnel significantly improves the traffic throughput to and from south of Stockholm as there are only two tracks in that direction from Stockholm Central Station, the same number that were in place in 1871 when the railway was originally built. It has 24 scheduled trains per hour in each direction. The commuter trains pass Stockholm with up to 16 trains per hour per direction. The other eight are regional and long-distance trains. The tunnel takes all commuter trains, allowing more regional and intercity trains to operate along the old line.

Wikipedia