Do you know of any study about ambient music on public transport?

I am of the intuitive opinion that soft relaxing good quality ambient music on trains etc. is a good thing to make into a norm.

I believe it reduces tension and ultimately will reduce violence, assaults etc. not to mention, to a certain degree, reducing the urge of some people to put music blasting from their cellphones.

I have been discussing this with friends and would like to sustain my feelings on some hard data.

If you know of anything, please let me know, and feel free to let your own personal opinion be heard.