I had 2 pairs of Grove dual trumpet 1609 air horns under the cab of my F350. Fed them with 3/8" line and had 3/8" solenoids mounted directly to the horns and 5 gallons of storage to supply them. They sounded great and would do that setup again in a heartbeat.
Train horns are nice, but to get good long blasts, you need upwards of 10 gallons and feed them with 1/2" air line. They use up ALOT of air. Another downfall is that they are bulky to mount.