In September when I was out in NG country we rode the D&S to Silverton.
I asked the brakeman(Who got off to protect the rear every stop) why they used hand signals. He said they do both, hand signals, and radio to be both more save, and for tradition.
On the C&TS I am not sure what the rules are, but I am assuming its similar, or maybe just radio most of the times. However, C&TS has been known to use their whistle signals for as long as I can remember, both in the yard and on the trains. A unique feature is the blowing for each station or location ahead. The engine blows once to call out being close to the next potential stop. After a pause the engine will blow two times to signify they are not stopping, and 3 times if they are. You here this each time as they approach Lobato(just before they pass Lake Lobato). Usually its two whistles for a single engine, cause they can cross the bridge, but if its a doubleheader you will hear 3 whistles first on the lead engine, then the second engine will repeat it as an acknowledgement.
The crews repeat this for places like Cresco,Cumbres, etc, etc.
Whistles are more prominent in the United States than foreign countries, since we are dealing with vast distances, and lots of highways, private roads and so forth. Overseas in Europe for example much of the right of way is enclosed and you hardly hear whistles. On my first visit to Europe in 1972, Germany employed a bunch of folks just to be crossing gate guards who would lower a gate as a train approached. Much of the ROW was enclosed like that.
Greg