Richa Singh
The semaphore was the first railroad signal to use electricity. The original semaphores used a two-position arm/blade to alert train operators.
They would hold three types of flags: red, yellow, or green. Those flags indicated how far away the passing train was and if it was safe.
In 1841, the UK started to use mechanical signals, instead of holding flags. The Bobbies became lookouts who reported train activities to signal box operators.
The railway design started to use blocks to ensure a clear path for each train.
In 1860, the system started to include a yellow warning sign, considered a distance signal. It lets drivers know they must slow down; the next signal will be red.
Initially, British Railways continued with, and developed, signals of the Big Four constituents.