Gamla Stan means the old town.
You can access this old town from T Centralen by metro and commuter train (Pendeltåg in Swedish) and any public transportation.
I walked from T Centralen, Gamlastan, Djurgården to Kungsträdgården.
When you go from Gamlastan to Djurgården, you can use the 82 ferry operated by SL with only 37 SEK.
This is very short but the most beautiful walking trail in Stockholm regardless of the weather.
I was waiting for the rain to walk on the old street in Gamla Stan to feel the old atmosphere.

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