Suffering diffculties with the public getting of the tracks I have mounted signal horns. Not the usual small electric horns of a car, but compressor-driven horns of a truck (ok, only a two-chime horn - should be a three-chime, I know). The horns are 15" and 12" long, with an almost prototypical low frequency sound (in comparison to the usual small car-horns).
The horns and the compressor are mounted in the rear of the shell's frame where the prototype's radiator would be.
View from downside of the shell with the compressor next to the horns - the system will act either as a melodical horn or produce two tones simultaneously (the option which I have chosen/wired).