"When changing to the Piccadilly line from the Jubilee/Victoria (after that long long walk) at Green Park you go over the Westbound line before descending the stairs.
At that twist/turn, the passageway is blocked off by a grille. On two occasions, I've walked past and the lights are on in the passage (much like sometimes the maintenance lights are on in the train tunnels themselves). Despite this, I still can't see the end of the passage. It's at least 20 yards long if not longer. For a while I thought it would lead to another platform, maybe going to Aldwych.
But of course, the old tube maps say that branch line started at Holborn (where you can see the links to the old platforms). So what's down there?