In the 1950s, how is a message sent from a speeding train via "loop"?

Post date: 2022-11-28 11:07:21

In the 1952 movie The Narrow Margin, the police officer hero sends a message from a speeding train, apparently by writing it on a piece of paper for a railway employee next to the track to physically swoop it down. What is this device/interaction/method of communication ?

This all happened very quickly, so it was hard for me to understand what was happening or know what I was looking at. This link briefly describes the shot: https://classicmovierev.com/the-narrow-margin-1952/

" At the next town, a railroad employee snags a loop off the speeding train with his arm and has Brown's message for the Highway Patrol."

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