As someone who promotes transportation choices for a living I greatly admire and feel a little bit envious of what a great job the New RailVolutionaries, a national organization of young transit professionals, did with the Streetcar Mobile Music Festival.
The music festival happened entirely on the streetcars, while they were in service and open to the public. Imagine walking onto to your train and stepping into the middle of a live performance! A local portland video company produced a fantastic 10-minute video highlighting the event. If you don't have time for the full 10 minutes, just watch the first minute or two to get the feeling of the event. It's worth it.