This signalbox was situated on the West of England Main Line. The frame has 5 bar tappet locking at 5¼" spacing, converted from a twist frame. The layout was much extended in 1931/32 and the dog chart is dated 15.4.1937, presumably the date of conversion from a twist frame. The dog chart shows the position after lever 22 had been made spare - the No.1/No.2 siding points becoming hand operated at this time. Originally lever 43 was an outer distant but following closure of Trenance Junction signal box and singling of the branch on 13.11.1949 it became a disc controlling a backing movement onto the branch. The locking remained unchanged when disc signal 20 (at 21C points) was removed and through subsequent changes in March 1965 (trailing siding connection onto Down Main through points 32, opposite disc 34/43 with discs 31 and 33 controlling movements out of and into the sidings removed and levers bolted normal), July 1965 (when Par Harbour signal box closed, leaving the sidings there to be accessed via St Blazey/ Par Bridge), September 1966 (when Trenance Sidings signal box closed) and May 1968 (when the branch at Trenance Junction was closed).
The box diagram therefore shows the position as at 1968, working to Par Station and Burngullow boxes, with a 'Limit of Shunt' on the Up Main well into the block section, not far short of the point where the branch once diverged at Trenance Junction.
Although some small changes have been made to the layout at St Austell since 1932, the interlocking has pretty much stayed the same from 1937 to 1968, with the exception of lever 43 - changed from an outer distant circa 1949 to provide access to the branch at Trenance Junction.
Running signals are easy to interpret from the box diagram - standard principles apply and their interlocking can be readily inferred. Shunting signals exhibit much more 'craftsmanship' in their interlocking, reflecting the practices of the relevant period.
The mechanical locking table and or dog chart illustrate this - particularly on levers 8 and 14. There are four disc signals on the Up Main, each reading only over the adjacent connection reverse - signals 15, 20 (now removed but originally at 21C points), 25 and 34. A move from disc 8 or 14 along the Up Main requires one (and only one) of the disc signals ahead to be cleared, the others being passed at Danger, as necessary, in the direction to which they do not apply. G.W. practice was that all disc signals had a red face, varying criteria over the years determining whether they carried a clear or red spectacle (and whether a clear spectacle meant that the signal could be passed at Danger in some circumstances). A Driver's route knowledge was releied upon to tell him which shunting signals applied to any particular shunting movement.