Glasgow & South Western Railway block instruments
The Glasgow & South Western Railway developed its own instruments based around the principles of the Tyer's one-wire, two position block. However, this instrument achieved a third block indication by a small disc indicator adjacent to the miniature signal arm in the instrument.
I have not established exactly how this system operated, or was worked, but I believe it entailed one additional line wire. Any additional information on the operation of these instruments would be appreciated.
The construction of the instruments themselves is pure Tyer & Co - the case, the block bells above, and the bell plungers below - all mounted on a backing board.
The examples illustrated here were at Elderslie No.1.
Any additional information about the working of these instruments would be appreciated.