Modeling Modern Defect Detectors

Overall view of Conrail clearance detector in New Jersey.
Close up of sensor on pole for a clearance detector.
Conrail equipment cabinet housing electronics and radio equipment (note small antenna for radio transceiver).
Details West 235-900 Detectors, Hot Box
Details West 235-905 Detectors, Dragging Equipment
Details West 235-908 Electric Relay Cabinets, 5 x 7'
By M. R. Snell

One of the trackside icons of modern railroading is the defect detector. Known by various names such as HBD (Hot Box Detector) or DED (Dragging Equipment Detector), detectors can monitor one or more mechanical conditions on passing trains, thus making them an invaluable tool for railroads. With fewer employees on the ground to observe passing trains, these detectors are the eyes and ears of today's railroads.

The first detectors simply employed a trackside display of lights and a digital readout to alert the crew in the caboose of a problem with their train. With the advances in technology and the elimination of the caboose prerecorded messages are delivered via radio to the train crew. These messages usually start out by identifying the railroad, the location, and whether or not a defect is present. Other information may include the number of axles, the train speed, and the outside temperature.

On my own HO scale Conrail New Jersey Division I felt a necessity to include such a detector, as they are as much a part of the railroad landscape as signals and tracks. Modeling the actual physical plant of the detectors is fairly easy utilizing castings available from Details West. The real challenge lied in the audible part.

Hotbox and Dragging Equipment Detectors
Modeling the physical plant of a standard HBD-DED is fairly straightforward using Details West castings #235-900 for the hotbox detector, #235-905 for the dragging equipment detector, and #235-908 for the instrument shed which houses the electronics.

Install the castings as per the instructions, omitting the pole style signals. On most prototype installations the dragging equipment hardware is a rusty oxide brown and the hotbox hardware is silver. The instrument shed should be placed within sight distance of the installation, several feet off the track.

Turning attention to the audio, I was able to put the prerecorded detector message onto a sound module (with the help of George Solovay from Innovative Train Technology) with a speaker mounted under the detector. The prerecorded detector message is tripped by a track shunt. When locomotive wheels cross over the shunt the push button circuit is closed and the sound module will be activated.

To cut the shunt into the track you must first cut and isolate a 1.5" section of rail. After the cuts are made, isolate the section using strip styrene, glue in with CA type cement, and then file to match the contour of the rail. In the opposite rail now you will need to cut and isolate another section. This section should be slightly offset from the first. If it is directly opposite then any lighted car or resistive wheelset shall close the circuit and you will receive multiple activations of the sound module. The isolated sections should be of a distance that the front left wheel of a four-axle diesel truck should sit on the first isolated section and the rear right wheel of the same truck should sit in the opposite section. Allowing for a little play the shunt will be activated by both four- and six-axle diesel locomotives.

Once the sections are isolated, solder one wire to each rail and run these through the roadbed. These connect to the push button terminals on the sound module. The speaker should be installed under the layout in the area of the defect detector.

To add diversity to operations in addition to the standard "no defects" message if you wish to add a "hotbox" or "dragging equipment" message simply add a second set of wires to the shunt and use a second module with the defect message.

For use of a second message, rather than connecting the speaker directly to the module instead connect the speaker and DPDT toggle. The upper and lower positions of the DPDT toggle should connect to the speaker outputs on the modules and the center connection to the speaker itself. Mount the toggle on a dispatcher's panel or elsewhere on the layout and this will enable you to choose from either message.

Clearance Detectors
The second type of detector is the "clearance detector." With the advent of double stacks these have been appearing more frequently throughout the country along lines with obstructions such as low bridges. Placed well ahead of the obstruction they instantly alert the train crew and dispatcher of the height problem.

Clearance detectors are usually just two poles parallel with each other, one on each side of the tracks. On the top is a sensor that reads from pole to pole. When the beam is broken it activates the detector with a clearance defect message. With the lack of a clearance defect the detector usually remains silent.

To model this installation cut 2 sections of hollow aluminum or brass tubing. Cut a small piece of styrene and file to the contour of the sensor. The concrete bases that the poles are mounted in may be added using round styrene or castings.

In order to activate the module mount an infrared sensor in either the poles or in adjacent scenery. The sensor's electric "eyes" should be aimed at each other and at the height you wish the detector to activate at. A train of excess height will break the beam and thus set off the circuit.

The connections to the module are similar to those used for the HBD-DED but will vary dependent upon the infrared module you choose and it's specific wiring configuration.

Getting The Message
The audible messages transmitted for each type of detector vary from railroad to railroad. There are several sources and ways to record and create messages.

The first method is to find a detector along the line you model and take a scanner and a tape recorder. Place the scanner and recorder in a quiet area, such as inside a car, and simply record the detector announcement.

The second way is via the Internet. Several websites offer amateur recordings of talking defect detectors. I have listed two sites in particular at the end of this article.

The third option is to use a computer-generated voice to make your own recordings. Some word processors for Mac and for PC's have the option to read aloud text. Once you get the message tweaked to your liking, you can make a recording of it.

While most detectors are specific to list car counts and train speeds with a little editing of the recordings you can successfully tailor the recordings to fit your specific situation. One example is Conrail detectors. On my railroad I have added the detector near the town of New Market. The actual recording I have is "CONRAIL - NEW MARKET, NEW JERSEY - NO DEFECTS - TOTAL AXLE COUNT... 4-7-6 - TRAIN SPEED... 4-3 - OVER" With a little editing you can reduce this to "CONRAIL - NEW MARKET, NEW JERSEY - NO DEFECTS - OVER" and thus making the message accurate for any passing train.

This has proven to be an interesting and unique feature of my railroad and has added an interesting operational feature as well while being a minor investment. You can easily recreate the same effects on your home layout, while adding to the modern railroad atmosphere!

Innovative Train Technology
PO Box 5042
West Hills CA 91308
(818) 992-6124

Details West
235-900 Detectors, Hot Box
235-905 Detectors, Dragging Equipment
235-908 Electric Relay Cabinets, 5 x 7'

Detector Audio sites

Matt Snell, 35, born and raised in northern New Jersey, the basis for his HO scale Conrail New Jersey Division, is a dockworker currently residing in Milford, Ohio. Matt has been a model railroader and a railfan since age 12 and is currently married, "with goldfish and trains."