Well I never saw one of those things in working condition...however the reason (I heard) the tube was there was because the engine men could not hear exploding detonators in the cab, and as a result the engine could clean up a works gang because they failed to slow down in time.
On steam engines the crew had many jobs that might keep the fireman away from observing long enough to miss seeing the flagman. The tubes amplified the sound into the cab, making sure that the detonators were heard.
Am I right? as I said, never saw one in coal drag, although I did play on the one that visited Albury around 1989 And that apparatus looks a lot bigger than the ones on my Eureka Garratt

Cheers
Rod