Yes, learning how to "work the cord" to get a nice feathering whistle takes some practice--but some are easier than others. As I recall, the 19 had a valve that liked to slam shut so you had to learn to feel for when it would open, and then let up a hair to start the feather before the whistle has time to sound-condensate in the line helps a bit on that.
There's nothing like a whistle artist to raise the hairs on the back of your neck--so sweeet!
And the regulars can do it without hardly thinking about it, busy with watching the tracks ahead, water level, etc.