I don't know about the other routes, but on the Sand Patch Grade the horn glitches out and the sound gets "stuck" in the environment. If anyone else knows what I'm talking about and can articulate this issue better than I have, please do! This has been happening on and off since TSW1 and I'd really like to see it get fixed.
No, it's more complicated than that. This specific glitch, when the horn is depressed, makes it so that the horn is stuck to "on" but the sound stays in that area in the environment where it glitched. The further you move away from it, the quieter it becomes. The sound also pops up again several times on that route, usually a mile or two apart. Pressing the horn again does nothing to fix it, unfortunately. I wish I had a video file that I could upload so that it made more sense. The ONLY way I've figured out how to correct this is to save the game, exit to the main menu, and then go back in.
I think it's a well known issue. I've certainly had it happen on several occasions. And like you say, the sound gets stuck in a fixed location. You can literally move the camera closer or further away from the point and it gets louder and quieter. Once this occurs, the horn button does nothing. The horn audio then just keeps reappearing as and when it chooses. Very annoying and ruins any enjoyment, and you just end up quitting. EDIT: I'm on PC too.
Yep, that happened to me with the C40 on the first scenario hauling a coal train. It started around Ellerslie and kept doing it all the way to Cumberland when I finished the scenario. (This was on PC though)
there's a way to fix the horn button not making any sound. just fly using the 8 camera far enough away from your train that you get a loading screen when you pop back into the cab. the horn will work after that.