The three core type-buckets
Train horns sold for trucks fall into three architectures, with everything else being a sub-category or an SPL marketing claim. Pick the architecture first, then narrow to the specific kit inside it.
Electric (single-piece, 12V, no air supply)
An electromagnet vibrates a steel diaphragm with a spiral resonator amplifying the output. No compressor, no tank — drop-in factory horn replacement. Physics-capped around 140 dB peak. Stebel Nautilus Compact at 134 dB DJD-verified is the loudest verified pick. See electric train horn for truck for the full breakdown, or the closely-related snail train horn category for spiral-housing variants.
Air with compressor + tank (chord-class)
Real trumpet bells fed by 110-150 PSI from a dedicated tank, pumped by a 12V compressor (Viair 400C, Kleinn 6350, HornBlasters 1NM). This is the architecture behind every 140+ dB kit. Air train horn covers the category; with-compressor anatomy walks the four-component install. For the locomotive-grade subset see real refurbished Nathan and Leslie horns and freight chord vs Amtrak chord.
Air without tank (tankless or wet-tank-tap)
Two paths skip the tank: pneumatic tankless uses a 12V direct-drive compressor pumping air directly through trumpets (Kleinn Direct Drive 6126/6127 is the only mainstream option, ~131 dB ceiling), and no-compressor builds tap the factory wet tank on Class 8 semis or use CO2 cylinders for show trucks.
Marketing-claim categories
Three search intents are about specific dB numbers rather than architectures. 300 dB train horns are physics-impossible (atmospheric ceiling = 194 dB). 150 dB train horns are methodology-soft — the real number is 147.7 dB DJD-verified on Shocker XL. 12V train horns covers the power-system constraints (compressor amp draw, fuse sizing, alternator headroom).
Specialty subcategories
Train whistle horns are steam-era brass replicas, audibly different from chord horns (single tone or simple chord, 116-134 dB realistic). Train horn kits walks what's actually in the box across budget, mid, premium, and locomotive tiers.
Once you've picked the type, narrow to the specific kit at best train horn kit or chassis-specific picks at by vehicle.