This was just a thought I had while playing a scenario today. RW already penalizes you for using the horn at the wrong times but it never rewards you for using it at the right times so I was thinking they should add proper horn usage to the score you get. Like performing the crossing sequence at all railroad crossings and using the right signals when you're preparing to back, releasing the brakes, etc.
I know a big problem is most people don't know what all the horn signals are, but that's why we have the internet: http://en.wikipedia.org/wiki/Train_horn#United_States