This is a diy train horn that anyone can make at home if they have a 3d printer.

THINGS YOU'LL NEED:
1 inch inside diameter 1.3 outside diameter pvc
painters tape or something similar like duct tape
1/2 inch flexible tubing
air compressor
#6-32 x 1/2 inch screws and nuts or something that is about 3.4 mm in diameter and 12 mm or longer with matching nuts
(optional) glue that bonds to your plastic and pvc
(optional) T fitting for more than one horn
(optional) mount for horns http://www.thingiverse.com/thing:1484130