The London South Western Railway (LSWR) first started using pneumatic control, rather than traditional wire and point rodding, at Grateley in 1901, utilising the system from the British Pneumatic Railway Signal Company that used low pressure air at 15psi to operate the signals and points. Following this successful trial Salisbury station, that was in the process of being rebuilt at the time, was also so equipped with the two new boxes, Salisbury East and West both containing a 64 slides frame (slides replaced levers on such system).  Air compressors and reservoirs were located next to each box. The system was brought into use in November 1902.  Unlike the systems installed at Grateley (1915), Baskingstoke to Woking (1966) , Staines (1930) and Clapham (1936) the Salisbury system remained in service until 1986.

The two signals on Fisherton Sarum are based on the down advanced starter and the up inner home bracket signal for the approach to either platform 1 or 3.   Andrew Hartshorne proprietor or Model Signal Engineering, from photographs of the actual two signals, kindly provided me with the correct combinations of his kits and parts to most economically make up the signals. I have also included a representation of the air cylinder mounted just below the balance arm.

Each signal is operated via a Viessmann stall type motor mounted directly beneath the baseplate that the signals are constructed on and this allows the signal to removed from the layout for maintenance if required.  Unfortunately it appears that these motors no longer are available and I would therefore think about using a slow motion stall motor type turnout motor in the future.  As the mentioned above the signals at Salisbury were pneumatically operated and therefore did not exhibit any ‘bounce’ so this was not incorporated into the control system.
I think it always good to see working signals on any layout, although a little fiddly to build the effect is well worth the time taken.

Each signal is also interlocked with a track section next to it to prevent any passing of the signal at danger. On Fisherton Sarum it is usually the fiddle yard operator that drives trains towards them so there is a switch on each of the fiddle yard  local panels to operate the signal. This switch is duplicated on the main control panel with a ‘local /remote’ switch to dictate which of the switches operates the signal.

For those interested in finding out more about the low pressure signalling system I suggest readers join the South Western Circle as following an excellent talk at one of the Society meetings earlier this year, Stuart Isbister has agreed to write a Circle Monograph on the subject. As the Monographs are issued free of charge to members, in addition to the quarterly Circular publication, they are usually worth the membership fee alone!