I have been licensed for more than fifty years, and I have never known the origin of the term "lid." This explanation clears things up. I did know that tobacco cans were used by news wire services and others to amplify the volume of the sounder. Now I know the other reason. Long ago, I picked up a tin can at a flea market and later found out it was one of the "preferred" tobacco cans to use.

73
Ken N2UK

The tobacco can was mounted to the telegraph sounder as a primitive yet effective means to amplify the clicks. These telegraph systems were a party line with many operators sending and receiving "traffic." The receiving operator would copy (listen for) a message header directed to them, otherwise it was just background noise.
When a poor "fist" (sending operator) came on line and the traffic wasn't for them. they would put the lid back on the can so they didn't have to listen to the novice operator. Hence the term "Put a lid on it!"

In the Ham Radio community, a poor operator today is referred to as a "Lid."

For a railroad environment, that's pretty much a white-collar job, and he's dressed for it, at least for the first half hour or so, until the grime and sludge start to slide off the walls and work surfaces. Turn me loose in there with a bottle of Windex and a roll of paper towels ...

Always the ubiquitous empty tobacco tin on the Morse code sounder, to amplify the sound.