Why Do Train Horns Face Backwards? Locomotive Horn Placement

If you’ve ever stood at a grade crossing and noticed that some of the bells on a freight locomotive’s horn point toward the rear, you’re not imagining it. The short answer to why train horns face backwards: a freight engine has to warn people in both directions, and it often runs long-hood-first or shoves cars in reverse, so railroads turn one or more bells around.

The Short Answer: Locomotives Run Both Ways

A truck almost always drives forward. A freight locomotive doesn’t. Units get coupled back-to-back, run long hood forward, switch cars in yards, and shove trains in reverse. Whatever end is leading, the people near the tracks still need a warning.

Trains magazine’s Chris Guss explains the practice directly: with a multi-bell horn, freight railroads “would typically reverse one or more bells to provide warning toward the rear of a locomotive when it’s operating backward.” A later Trains.com explainer puts it the same way. Freight locomotives carry “a mix of the five bells facing forward and reverse” for protection in either direction. Passenger locomotives typically keep all bells pointed forward because they rarely run in reverse for long.

• Freight units: some bells forward, some reversed, because they lead in either direction
• Passenger units: usually all bells forward, because they rarely run in reverse for long periods
• Some older fleets: two separate horns, one at each end

Reversed Bells vs Two Horns: The Options Railroads Used

Turning a bell around isn’t the only way to cover the rear. Per the same Trains article, railroads that kept every bell facing forward would often add a single-tone horn, or sometimes a second multi-bell horn, for warning when operating in reverse.

Trains also noted that some former Southern Railway locomotives were still in service on Norfolk Southern with two horns, one mounted at each end. On those units only one horn works at a time, and which one depends on the position of the reverser handle in the cab. In effect, the horn follows the direction of travel.

The reversed-bell approach is simpler: one horn, one air supply, and sound going both ways. A train horn makes a chord, so you can reverse just one or two bells and keep the forward-facing bells doing most of the work toward the front. If you want to know how the bells and diaphragms actually produce that sound, our explainer on how train horns work, from compressed air to chord covers the mechanics.

How to Read the Model Number: The “R” Suffix

Railfans often identify a reversed-bell horn by the letters after the model number. The clearest published decoder is for Leslie Supertyfon horns. A reference page on the Alaska Railroad Photographs site, covering a Leslie S-5T, lists these suffixes:

R (suffix)

One bell reversed (turned backward)

RR

Two bells reversed

O

Some bells swapped on the manifold vs the standard layout

F

Full openings in orifice dowel pins

Their example is an S-5T-RRO with the #31 and #55 bells reversed.

Nathan Airchime horns show up with similar suffixes. A federal test report describes the K-5-LAR24 as the standard K5LA “with 3 chimes facing forward and 2 chimes (370 and 494 Hz) facing rearward.” The same report lists a Leslie RS-3L-RF as an RS-3L with one chime (440 Hz) facing rearward. A Trains photo caption also shows a new Canadian National ET44AC carrying a Nathan K5HLR2. Treat the exact meaning of each letter-and-number combination as model-specific. For a closer look at the two most common Nathan configurations, see our Nathan K3LA vs K5LA comparison.

Why the Horn Sits in the Middle of the Roof

The second half of the puzzle is location. On many modern freight units, the horn isn’t on the cab. It’s partway down the roof, and that placement has a crew-comfort origin.

A study by the Volpe National Transportation Systems Center for the FRA notes that in the late 1980s it became “the de facto standard to install horns on the top/center portion of the locomotive,” in an attempt to reduce noise exposure for the people in the cab. Trains.com says the same thing in a photo caption: horns are typically placed near the middle of the locomotive to minimize noise to the crew.

The Volpe data shows how much that helps. Moving the horn from the cab roof to the center of the locomotive cut interior cab levels by 3 to 11 dB(A). Closed windows added another 5 to 15 dB(A) of reduction.

A center-mounted horn has another advantage. Sitting near the middle of the unit, it isn’t stuck at one end, so a reversed bell has a clearer path toward the rear.

The Trade-Off: The Locomotive Body Blocks Sound

Center mounting has a cost, and the federal tests measured it. Volpe mounted five horns in four locations (center, cab roof, front hood, and above the coupler) on an EMD GP-40-2 and an EMD SD60MAC. Then it measured the sound around each locomotive and inside the cab.

Horn	Locomotive	Horn distance from front	Forward reduction at 100 ft from horn
K-5-LA	GP-40	30 ft	-10 dB(A)
K-5-LAR24	GP-40	30 ft	-10.8 dB(A)
RS-3L	GP-40	30 ft	-9.8 dB(A)
K-5-LA	SD60MAC	40 ft	-18.3 dB(A)

Those numbers compare a center-mounted horn with the same horn on the cab roof at the same height. The locomotive’s own body throws a sound “shadow” ahead of the unit. The study found that the forward levels of a center-mounted horn ran 8 to 10 dB lower on the GP-40 and 15 to 18 dB lower on the SD60MAC than forward-mounted horns at equal air pressure. The SD60MAC fared worse partly because its horn sat 40 ft back with only 1 inch of clearance above the top of the locomotive, against 9.5 inches on the GP-40.

Volpe’s conclusion was that the cab roof best met all the criteria. Where a railroad prefers a center installation, the report recommends mounting the horn “as far forward and high as possible” and forward of rooftop obstructions.

Union Pacific learned a version of this decades earlier. According to UtahRails.net’s history of UP locomotive features, the railroad moved horns from the cab roof to above the radiator fans starting in the mid-1960s to stop ice and snow from disabling them. Then studies in the early 1970s found that the rear location “reduced the horns’ effectiveness” because the body and roof obstructions deflected and masked the sound.

The problem came back with taller locomotives. Trains reports that the higher hoods on Tier 4 emissions locomotives kept enough sound from reaching the rear of the unit. GE’s demonstrators carried two horns, one above the engine and one at the rear of the long hood, before the production design returned to the standard Nathan placement.

What the Federal Rule Requires

The U.S. rule doesn’t dictate which way each bell points. Under 49 CFR 229.129, each lead locomotive’s horn must produce at least 96 dB(A) and no more than 110 dB(A) at 100 feet forward of the locomotive in its direction of travel. For most horns the test microphone sits 15 ft above the rail. For cab-mounted and low-mounted horns it drops to 4 ft. Trains notes that locomotives must be tested in both directions of travel, which is one more reason a freight unit needs usable sound off both ends. Our full breakdown of the 49 CFR 229.129 locomotive horn rule covers the test procedure.

Placement matters under the rule in one more way. A locomotive that gets a replacement horn can skip retesting only if the new horn is the same model and the mounting location and type of mounting are the same. Move the horn and you’re testing again.

The rule’s history shows placement was debated directly. The FRA’s 2005 final rule record notes that Caltrain had relocated its horns to about four feet above the rail. After railroad comments, the FRA added the 4-ft microphone height for cab-mounted and low-mounted horns.

Canada is more prescriptive. As summarized on Wikipedia, Transport Canada’s locomotive design requirements call for horns mounted facing the direction of travel, near the front of the roof, no more than 1.5 m behind the rear of the cab, and near the centerline.

Does Any of This Matter for a Truck Horn?

Not much for the “backwards” part. Your pickup leads with the same end almost every time, so there’s no reason to reverse a bell. The placement lessons do carry over:

• Aim the bells where you want the sound to go, because the horn body and vehicle panels block what’s behind them
• Keep bells clear of obstructions in front of them, just as Volpe recommends mounting forward of rooftop clutter
• Higher is generally better over soft ground: Volpe measured an 8.4 dB drop per doubling of distance for a horn 3 ft up, vs about 6 dB at 16 ft

• Don’t bury trumpets behind a bumper or skid plate and expect full output
• Don’t copy a locomotive’s rear-facing bell layout on a vehicle that only needs forward warning

For truck-specific spots and angles, see our guide on where to mount a train horn on a truck.

Keep reading

• How Do Train Horns Work? Complete Explanation
• Locomotive Horn Rules: What 49 CFR 229.129 Requires
• Nathan K3LA vs K5LA: Which AirChime K-Series Sound Fits You?
• Nathan vs Leslie Train Horns: Which Locomotive Horn Sounds Better?
• Where to Mount a Train Horn on a Truck: Best Spots & Angle

Sources

• All about air horns, by Chris Guss (Trains.com): freight railroads reversing bells to warn toward the rear, single-tone and second-horn alternatives, Norfolk Southern ex-Southern two-horn units tied to reverser position, center placement to minimize crew noise, CN ET44AC with K5HLR2, testing in both directions, Tier 4 hood-height problem and GE two-horn demonstrators
• Locomotive horns are important safety features, by Chris Guss (Trains.com): freight mix of forward and reversed bells, passenger locomotives with all bells forward
• The Effect of Installation Location on Railroad Horn Sound Levels, Rapoza & Fleming, Volpe Center (USDOT ROSA P): late-1980s top/center de facto standard for cab noise, K-5-LAR24 and RS-3L-RF bell configurations and frequencies, test locomotives and locations, installation-effect table, 8-10 and 15-18 dB forward reductions, 1 in vs 9.5 in clearance, 3-11 dB(A) cab reduction, 5-15 dB(A) window reduction, ground drop-off rates, “as far forward and high as possible” recommendation
• 49 CFR 229.129 Locomotive horn (Cornell LII): 96-110 dB(A) at 100 ft forward in direction of travel, 15 ft and 4 ft microphone heights, replacement-horn exemption requiring same model and mounting location
• Use of Locomotive Horns at Highway-Rail Grade Crossings; Final Rule, 70 FR 21844 (GovInfo): Caltrain horns relocated to four feet above rail, FRA adding 4-ft test height for cab- and low-mounted horns
• UP Diesel Locomotive Features: External and Internal Features of UP Locomotives (UtahRails.net): UP moving horns above radiator fans in the mid-1960s for ice and snow, early-1970s studies finding the rear location reduced effectiveness
• Leslie 5-chime Supertyfon Horn (Alaska Railroad Photographs): Leslie R, RR, O and F suffix meanings, S-5T-RRO example
• Train horn (Wikipedia): summary of Transport Canada horn mounting requirements (facing direction of travel, near front of roof, within 1.5 m of cab, near centerline)