Since a few days, the TTN "Central Railway Station" gateway in the inner city of Berlin is on-line 365/24.
Thanks to activities of @RecumbentTravel we are getting some more realistic real live data to form a big picture.
Today a 4 km connection could be established (see picture).
So a radius of about 4 km may cover the whole inner city of Berlin with TTN.
Of course, still more data are needed. But it's a good starter, anyway.