So I checked the map to find a suitable destination on the line through the tunnel and found my way to the platform.
Saltsjöbaden is a small suburb some distance from the city centre, it is on the end of a penninsular, and hence, on the end of its train line (though apparently the line did have a further station on it, but I can't figure out where the track would have gone). The station is by the Grand Hotel, an impressive waterfront hotel with ample yatch parking.
There are a number of small islands here, some connected to the mainland with small bridges, in some cases only allowing pedestrian access, and in at least one case the bridge was bigger than the island (I'm not sure whether it is tidal here, but I don't think so, theres not much evidence of continuously varing sea levels).
Having found the landing point of the ferry back to Stockholm I decided not to catch the 2:15 ferry and instead wait for the next one. Again, the wrong thing to do since the next was three hours away. So I took a little while to explore around the neighbourhood, which seemed to consist mostly of ostentatious houses perched on rock outcrops.
Boredom set in at around the same point my hands started to freeze so I figured it was time to get the train home rather than wait for the next ferry.