Six Mile Bottom signal box in November 1975. This picture shows the block instrument for the Fulbourne - Six Mile Bottom section. The bell (gong) is seen above it. The block instrument received a bell code from Fulbourne asking Six Mile Bottom to 'accept a down train' or, in the case of an up train, the instrument would send an 'accept' request to Fulbourne. Bell codes between the two boxes were repeated in each box. Once a train has been 'accepted' by either box and signals set accordingly, the pointers would then indicate a train is 'in section' ('line clear’ being the other indication). Once a train was 'in section', the signalman then had to contact Dullingham via a second block instrument to see if he would 'accept' the train (or vice versa). Only then would the signal for the section to Dullingham be 'pulled off'. As a train cleared one section, the respective signalman then had to notify the preceding box that the train is 'out of section', again via bell signals. All these actions then had to be recorded in the box's log book. At locations with a level-crossing, such as Six Mile Bottom , the barriers could not be lifted until a train was 'out of section' and signals reset to 'on'.
Photo by David Burrows from his Flickr photostream