‘Greenlight’: How a term for trains has become the go-ahead in any trade

livemint.com:

When actions need approval or authorization, someone is usually waiting for a “green light." “Fed Gives Green Light to Lowering Interest Rates," blared headlines after a recent speech by Federal Reserve Chair Jerome Powell. Other papers reported that season four of the Apple TV series “Ted Lasso“ is “Close to a Green Light." The latest Covid-19 vaccines have received a “green light" from the Food and Drug Administration. “Green light" is also part of the colorful metaphorical language exchanged between writers and editors.

It is a term that I’ve grown to appreciate over the 11 years that I’ve been writing this column, which ends this week. My weekly routine always began with a topic I proposed to my editors—“a pitch," to use a metaphor borrowed from cricket and baseball. When I got the “green light," it was full steam ahead.

Steam is actually key to understanding the roots of the “green light" metaphor, which began in the days of steam-powered locomotives. In 19th-century Britain, engineers needed a system of signals to regulate train traffic. While red lights were always a signal to stop, the “clear" signal was originally a white light, while green signified “caution." As an 1839 guide explained, “A green light should be placed at each station at the spot where the engine-man should slacken his speed." It soon became clear, however, that white wasn’t an ideal “go" light.

Engineers found that if the glass on a red or green light broke, it would appear white, making accidents more likely. So green took over as the signal to proceed, and yellow or amber was used as a signal to slow down. With the rise of automobiles in the early 20th century, electric traffic lights with green, yellow and red lenses became a standard