Bill, in the UK these are (or were) called warning detonators. One time when I was a small child my fathers train de railed leaving it obstructing the adjacent line which was used by the London Edinburgh express. He had to run two miles up the line to place the detonators to stop it. My father didn't talk much about anything in detail he just said to me, no one tells you how to know you have actually run two miles, The train de railed on a curve and the detonators had to be fixed so he didn't know how much time he had, instructions were a minimum of three, and if you saw the express coming it was too late to start putting them on. Since they drew the attention of a driver on an express at full speed I am surprised they didn't blow a small bogie off into the next weekend.
Anyway Bill Happy Birthday.