The ball signal was a very, very early signal on American railways. When hoisted up was proceed and down was danger; so i suppose very early failsafe.
It featured in the TV western episode of Cimarron Strip, "The Blue Moon Train". The series was set in 1888 but I wonder if it was still in use.
It was the derivative of the term "Highball" for an
American Express train.
Old photos of early
GWR▸ signal boxes show the signalman exhibiting a white flag to passing trains in addition to the fixed signals.
Facets of the old red-green-white continue to this day. When shunting at night using a hand lamp on Britain's railways a white light is used for normal shunting and a green light for shunting at reduced speed.
On the Isle of Man Railways, on departure, the train complete signal from the guard at the rear of the train is a yellow flag. On checking the old rule book this is a substitution from the original white flag specified. White railway flags are not made anymore and they are difficult to keep clean!
PS 'Signaling' is the American spelling!