In order to bring more live to the game, it would be really cool if AI could whistle, sound the horn or sound the bell at appropriate times. American trains should sound the horn upon departure, do "14L" horn signals at level crossings and turn on bells in yards. British and German trains should whistle at whistle boards. In terms of implementation, this feature should ideally be handled by fully automated trigger points on the track, so that scenario creators don't have to include separate instructions for every time horn, whistle or bell use by the AI is desired. I don't have too much technical insight into the game, but this feature shouldn't be too complex to implement and would greatly enhance realism and immersion, particularly on US routes.