THE BLOCK SYSTEM
Great Northern Railway instruments
GNR Three-Wire Three-Position block instruments
Unlike many of the impoverished pre-grouping railway companies, the Great Northern does not seem to have made any attempt to economise in the cost of telegraph wires by using one-wire instruments. Instead, they created their own style of three-wire instruments, many of which were built out of old telegraph instruments.
The instruments are arranged in pairs, with two sets to serve the sections to either side of the signal box. The larger instruments are pegging instruments – straight conversions from a telegraph instruments by providing a pistol-grip trigger device to hold the commutator to the left or right.
The smaller instruments are the non-pegging instruments controlled by the adjacent boxes.
Between the two instruments is the bell for communciation, neatly encased in wood.
The examples illustrated are at Blotoft Siding, on the Great Northern & Great Eastern Joint line. If you look at the picture carefully, you can see a BR block instrument behind them, awaiting installation.