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QUESTION: I would like to know if the medium of transmission is a steel rail ( i.e. train rails) and the method of transmission is frequency modulation,  whereby circuits are separated by frequency, is it possible to have destructive and constructive interference patterns?

ANSWER: Not sure I understand the nature of your question but I believe you are asking if physical modulation of railroad tracks can be destroyed with fm forces as opposed to beat frequencies of normal oscillitory disturbance.

The Tacoma Narrows Bridge was destroyed when a wind force started it into a destructive motion.  See the youtube of actual failure.

http://www.youtube.com/watch?v=j-zczJXSxnw

But the Tacoma bridge was not stimulated by beat frequency.

The question of a railroad track being sent into oscillation is a strange thought since in this case the tracks are usually very strongly attached to the railroad ties - every few inches and the destructive force to put the track into oscillation or any regnerative destructive force is nearly unimaginable.  

What is your fascination with frequency modulated beat stimuli?  Is there something I don't understand or comprehend in your question?  Let me know more and we can discuss further.



---------- FOLLOW-UP ----------

QUESTION: perhaps i wasn't clear. Please allow me to expound. i work in the railroad industry. in this industry, train detection is acheived by means of track circuits along the entire rail system. These track circuits consist of a transmitter,  transmit  impedance bond; a receiver and a receive impedance bond. The impedance bonds represent the boundaries of each track circuit ( they block ac modulated track ckt. frequency and pass the 750vdc used for propulsion ). A total of eight audio frequencies  are modulated and  rotated throughout the system. when a train enters a ckt.  the transmitted signal does not reach the receiver because conditions for a valid track ckt. aren't met ( correct amplitude, frequency, modulation rate) and the train is detected.
  These impedance bond ( that represent the boundaries of track ckts.) are not brick wall filters to ajacent track ckts. They allow some bleed over ( to allow continuous travel for train. ) up to 10 feet into adjacent trk. ckt.
 My question is, since we are able to adjust these track circuits, if we adjust them to a level that intrudes into an adjacent trk. ckt. beyond the acceptable level, could it cause beat frequencicies by combining with the adjacent trk. ckt. frequency ( amplitudes in excess and amplitudes that are diminished). in either case the track ckt would fail safe but i'm just trying to come up with a theory as to why.

Answer
Thanks for your clarification.  It helps me understand more clearly. Unfortunately, whilst I have an understanding of what you are explaining I do not have any experience in that industry nor the testing and circuits that are used for gathering the outputs of the transmitters and receivers.  But, I do know the basis of the system you are using is centered around the resistance of the rails, a wheatstone bridge which has high sensitivity to the change in resistance of the tracks and comparisons of the signals as they are modulated.  I think they are all low frequency measurements and at insufficient levels to cause any deleterious results.

I don't know about the cross talk measurements from adjacent, parallel tracks and off shoots where there may be another train heading to the same junction.  It is interesting to me and I hope you can get a better explanation and let me know the result.

Hope this helps a little.

http://en.wikipedia.org/wiki/Track_circuit

http://greatergreaterwashington.org/post/5068/how-track-circuits-detect-and-prot

http://www.g6fsp.com/irsemrs/TC01%20v0-1%20DC%20track%20circuits.pdf  

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Electronics questions about AC, DC and digital theory.

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Graduate electrical engineer with over 40 years in electronic design, manufacturing, project organization and patent review. Experience in fields of industrial and consumer electronics (audio, video, acoustics, etc.)

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IEEE (Institute of Electrical and Electronics Engineers); Senior Life member AES (Audio Engineering Society), Fellow Life member

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BSEE University of North Dakota

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