It is irritating that it is closed, but I actually prefer getting a Thameslink into London these days, much more convenient for going up North from St Pancras, and is usually more useful for things to do in London.
Mystery solved (well mine anyway). Trains from the south terminating at Balham (three stops short of Victoria), then "turning round" and becoming a different a different local train.
Balham having interchange with the Northern Line obviously.