Back when we had the Guard's Van on our trains, before radios, the guard would signal with a small green flag, "right o'way" to start the train and then again to signal he was onboard. If the train was so long or out of sight around a curve, he would blip the airvalve to signal "right o'way" and continue to signal "Guard Aboard" until he was seen from the Loco. These signals were easier relayed at night with a torch(lamp) and were
always acknowledged with a toot on the whistle.
Radios were a vast improvement but elimination and replacement with "Fred" was the answer
Chris
in New Zealand