With a first name starting with K?
The circle has always been 40, with the portals being 30 due to the closeness of concrete pylons holding up the structure above it.
When you say 'always', how far back are you going? I recall that 60 was the speed limit, except for Circular Quay - St James where it was 80? In the 1970s. City Circle times used to be around 12 minutes, aren't they closer to 18 minutes these days?
The 30 at Goulburn St was, in my recollection, introduced after Granville, when extra supports were added to the base of the columns.
I am happy to be proved wrong on all of the above, but I distinctly recall trains travelling at much higher speeds than 40 when I travelled around the Circle in the 1970s.