Thank you for sharing the photographs of the signal work at Truro. I remember it happening - the details are in a Western Region weekly operating notice, Saturday 7 - Friday 13 March 1992 and it says "The existing mechanical bracket signal 48/49 has been recovered and replaced by a straight post signal with 49 signal arm only. The subsidiary signal (48) has been permanently taken out of use". The 30mph PSR board was also moved by half a chain when the work was done.
A "kind of" precursor to this work happened in January 1988 when the connections from the down main line to the yard were ripped out, the bracket until then had a disc signal also. After this time, it left the main stop signal and subsidiary arm. I saw the calling on arm used a few times, notably one weekday lunchtime when the Glasgow mail had shunted to the down platform waiting for the Paddington train to pass. The down London HST was late, and there was a light 47 in front of it. So, to keep the 47 moving and accept the HST, the calling on arm was used for the engine, it stopped the country side of the crossover, dropped back on to the up line on Carvedras viaduct then ran through the up platform, the west crossover and regained the down main. Another time was a Saturday teatime in spring 1989 when there was a failure west of Truro, the Edinburgh to Penzance HST was in the down platform and the teatime Plymouth to Penzance unit arrived on the viaduct. The calling on arm was used for the DMU to draw up behind the HST and get the passengers off via the first door. Railway work that was all in a days work to the men on the ground.
Regards, Martin. Many thanks Martin for this insight to what happens on the real railway.