Yes sorry, perhaps I didn't compose my initial post good enough. When a TSR as been terminated early, how long does the SPATE board need to stay in place before it can be removed? (and I notice Tomnick's answer which seems logical)
Also, why does the magnet need to stay in place when a speed as been terminated early?
GB, the misunderstanding was mine. I do apologise.
The TSR boards remain in place but with the speed(s) spated, until the published withdrawal time. This will either be shown in the WON on a specific day, or will end at the end of the WON notice week.
The reason that the magnet remains in place is simply that it forms part of the TSR equipment and in poor visibility will draw the Drivers attention to it.
It is spated following Recomendations from the 1975 Nuneaton derailment. At that time the TSR boards were removed, but it was realised that their removal could mask a failed warning board, as was the cause, when a Driver assumed the TSR had been withdrawn earlier than published.
After that the TSR remained in place, albeit with a new raised speed displayed, and in later years the "Spate" board was introduced.