As part of the same funding package as the new Red line branches there's two more major investments in rail transit that are now practiacally decided on.
First is that Roslagsbanan (the narrow gauge railway getting new cars just a few posts above this post) will be getting a new tunnel running from Universitetet to Centralen via Odenplan. I.e. the current city-end terminal will close permanently (and the land there redeveloped with mostly residentials) and be replaced with a tunnel that goes a very different route and that reaches all the way to Sweden's main transit hub.
Cost: 7.1G SEK
As part of the deal the City of Stockholm commits to building 500 residential units (yes 500. not 5 000 or 50 000. 500.). Vallentuna municipality 5 650, Österåker municipality 7 020 & Täby municipality 16 200.
New map for that railway:
source
Second is Tvärbana Syd. i.e. ~ the Southern Orbital. An LRT line connecting suburbs on the Red Line subway and on the commuter rail line to Södertälje. A further extension eastwards from Älcjö to Skarpnäck or even to Älta or Värmdö have been talked about for years already and has large local political support in those areas. Will likely use the same cars, signalling, etc. as the current LRT line Tvärbanan. Even though no connection is currently planned. Such a connection has been suggested by politicians though and in the long term I think it'll get built.
Cost: 4.2G SEK
Huddinge municipality commits to building 18 500 residential units.
source