The AirZound horn is an excellent investment, I've put them on two bikes so far. Drivers (including large trucks and semis) can hear it in traffic, as well as a pedestrian with music louder than your motor blasting in thier ears.
It's also fun to 'race' with a train and hit the horn. The few times I've done it they've smiled, waved and honked back.
(Note: on a road alongside the tracks, with no intent of trying to cross the train's path. Don't ever try to do that.)