This sounds like system stutter. Your system is trying to playback the horn sound as well as access the hard drive or do other things at the same time.
You may also want to do some general system maintenance such as defragging your hard drive (not for an SSD), disable any background applications, especially your antivirus application from scanning the Trainz installation folder, and turning off Microsoft Windows Search, and Indexing.
If that doesn't do it, try decreasing the repeat delay rate of your keyboard. I had to do this on my Alienware laptop and see if that helps. The laptop keyboard was a bit different than a regular keyboard and had a faster repeat rate. You can do this through the control panel.
John