Heya Toots! - The Story of the Interlocking Horn

Anyone who has spent time in towers or gotten up close looks at CTC consoles or video displays may have noticed the presence of buttons (or icons) labeled horn or whistle or emergency horn.  Occupying the same general category of ancillary functions as point heaters, maintainer call lights and low air alarms the "Interlocking Horn" is a vestige of a time long past when communication was intermittent and unidirectional, but still serves a purpose to this day.

The concept of an interlocking horn is pretty simple and basically serves as a method for the dispatcher or operator controlling the interlocking to communicate with train crews or other personnel who might happen to be in the area.  As you may have already surmised the use of binary state sound generators to transmit information have been generally supplanted by radios and mobile phones, but interlocking horns can still be found mounted on present day relay huts installed up through the current day (2014).

 I first learned of interlocking horns in a discussion of Pennsylvania Railroad block operation where they were described as being used to stop trains in an emergency if the operator became aware of an unsafe condition, but the train had already passed the controlling signal.  The operator would sound the horn and upon hearing it either the locomotive crew or (more likely) the rear end crew would bring the train to an immediate stop and use a line side telephone to communicate with the tower operator.  The practice always sounded a bit dodgy to me as trains are a loud place to work and unlike a visual signal it is a bit harder to verify that an alertness signal is not being sounded.  Still, horns can be made very loud and with 2 or 3 people riding in the caboose it was likely that at least somebody would catch the signal that the bridge ahead was out.  Still, a quick glance through the current NORAC rulebook shows that interlocking horns are a bit more complicated than that with a vocabulary that approximates the old trainline communication whistle system that was ultimately supplanted by crew radios.  Here is the exact text of NORAC rule 18.

18. Horn Signals Used by Dispatchers or Operators
The following are horn signals that may be used by Dispatchers or Operators at interlockings or other designated points. The signals are illustrated by “o” for short sounds and “—” for long sounds.
SOUND INDICATION
(a) — All movements within interlocking limits must stop immediately.
(b) o o Normal movement may be resumed after receiving the proper signal or permission of Operator.
(c) o o o Whistle or Horn test.
(d) o o o o Signal maintainer must call the Dispatcher or Operator.
(e) o o o o o Electric traction employee must call the Dispatcher or Operator.
(f) o o o o o o Trackman must call the Dispatcher or Operator.
(g) A bell, horn, white light or oscillating white light at remotely controlled signals and interlockings indicates that any employee, except on moving trains, must immediately communicate with the Dispatcher or Operator.

 Long story short the interlocking horn was intended to stop train movements or get some sort of wayside employee to call the dispatcher or operator.  In my discussions with current operators and dispatchers I have found that in modern times the horns are rarely used in this capacity.  More often than not they are used to scare children or other trespassers off the tracks or perhaps to warn nearby employees of approaching train movements although in those situations a full time whistle/safety man is often on had to do the job.

Naturally the Pennsylvania Railroad made frequent use of interlocking horns due to their embrace of and electro-pneumatic interlocking plants.  With a source of compressed air on hand, it was trivial to attach a horn to the side of the tower and then run a wire to a button inside.  Due to the length of some interlockings one horn would not be sufficient to provide warning at both ends so multiple horns were used with one being placed on the tower and others being mounted to remote air compressors or other air handling equipment.

Horn buttons in towers could be mounted on their own as seen in OVERBROOK, but often times they were placed on a panel with numerous other buttons for lights or communications or special interlocking functions.

The most common horn configuration makes use of two bells, one pointed in each direction to provide maximum coverage.  Usually the horn bells are symmetrical, but there are some examples of the horns being asymmetric.

The most amazing thing is that the concept of the interlocking horn outlasted the interlocking tower era itself.  Like point heaters and maintainer call lights interlocking horns were mounted on CTC era relay huts and hooked up to the code line for dispatchers to sound remotely.  Still, this isn't entirely surprising seeing that portable radios were not reliable until the 1980's and dispatchers still needed a way to get MoW crews in the area of a relay hut to phone home.  Where electrically operated switch machines removed the need for an on site compressed air plant, "klaxon" type electric horns were employed. As you can see with this example at NORGE interlocking on the CSX Peninsula Sub the small single bell was clearly intended to communicate with trackside workers and not the train crew.

Insect infestation is a problem in Tidewater Virginia.

Unfortunately as these and other 1970's era CTC relay houses are being replaced the new CSX standard hut does not provide for the continued employment of interlocking horns.  Even in the 1970's use of interlocking horns was not uniform as can be seen with LEE HALL interlocking being provided with an slender truck style horn while only two miles away WEST LEE HALL was not provided with one at all.

Truck horn and exposed cloth and tar insulated wire bundles.  Classy.

 As technology marched on so did the design and employment of the horns used at interlocking.  In fact some could barely be considered horns at all like this loudspeaker device mounted to the relay hut of LEADENHALL ST interlocking in downtown Baltimore, MD.

Interlocking horns were not just the purview of the PRR or former Chessie system lines.  Here we see a relay hut at CPO-4 on the D&H Colonie Sub which had been recycled from CPF-130 on the Freight Main Line sporting a small, 100% external klaxon type unit.

 New Jersey Transit is perhaps one of the last believers in the concept of the interlocking horn.  Here is a 2003 photo taken of the recently installed ATLANTIC interlocking relay hut in Atlantic City New Jersey with a compact twin bell unit mounted squarely on the side.

NJT is also committed to the lineside telephone box.

A decade later we see that NJT is still at it with a similar unit installed on the side of the brand new (2013) DIVIDE interlocking in Pennsauken, NJ.  A similar horn was applied to the new JORDAN interlocking two miles to the east.

Yes, that is a new relay hut design.

I am sure if I scoured my records thoroughly enough I could find other examples of interlocking horns on other railroads, but I am pretty sure you get the point.  The takeaway is that the next time you are out shooting some classic signal, make sure you get a nice closeup of that odd looking horn on the side of the relay house.  They are yet another disappearing vestige of our signaling and communications past.