This is a script that you can use to disable those bloody train sounds once and for all. It's very simple to use and does not 'hack' any of the wow files. It simply creates the working paths for the sounds that get used instead of what wow uses.
Download the script and place it in your main wow folder. Extract the bat file from the zip and double click on it. It should work instantly. You will see a black command prompt window open up and disappear very quickly. This creates all the directories and place holder files that will disable the train noises.
Alternatively to downloading, you can create the script yourself. Follow these instructions to do so.
Open up notepad
Copy the script below in the code window
Click on File -> Save As
Navigate to your WoW Program folder
C:\Program Files\World of Warcraft for 32 bit Windows
C:\Program Files (x86)\World of Warcraft for Windows
Change the Save as Type to "All Files"
For the file name, type "damn_train_1.0.0.bat"
Click Save and Exit notepad
Navigate to the World of Warcraft folder (same place where you saved the file 4 steps ago)