Radio Information Question

[NSyorktown]

Don't want to appear to be too stupid here . . . . but on the Division Resources pages on this site, there are some radio frequencies that are labeled as "dispatcher  to train duplex" and "train to dispatcher duplex". 

Example: http://jreb.org/ns/index.php?topic=4487.0

What is a train duplex?  And, for that matter, a dispatcher duplex?

Is that the feedback from defect detectors, FREDs, other?

TIA

[Michael Knight]

Duplex refers to a two-way channel where each party transfers and receives on two different channels. An example would be a crew transmitting to a dispatcher over channel 48 while the dispatcher transmits back to the crew on channel 9. Thus, the duplex AAR channel 48/09, or Southern's channel 2.

Simplex is where both parties, whether base or field, transmit and receive  over the same channel. An example would be AAR channel 56/56, or Southern's channel 1.

[NSyorktown]

As a follow-up, what frequencies are used for the automated messages from defect detectors? 

Are they among the local "duplex"  listing or are they a completely separate frequency (systemwide)?

I ask because I listen to the road frequency in my area (using a Vertex -150 scanner), so get the cab-to-distpatcher audio but there must be another channel for this "other" information.

Thanks!

[Rockin Roller]

Detectors are on the main road channels.

[E.M. Bell]

Detectors are on the road freqs only.  They are spaced roughly 10 to 12 miles apart over most of the system.  These use rather low power radios that, depending on the location of the detector, may not reach out that far.  Case in point, from here at home I can hear detectors for 30 or 40 miles in either direction on both lines, EXCEPT the one at Burgin Ky.  Its maybe 12 or 15 miles away from me as the crow flys, and I can almost never hear it, but its in a low area. 

[NS145]

Just as an aside to nsboxcar's excellent response, the professional grade radios used by the railroads allow for split transmit (TX)/receive (RX) frequencies, unlike a conventional scanner.  You can sometimes pick this out when crews talk about changing channels and say "let's go over to 2222".  In this case the engineer wants the other crew members to change both their TX/RX frequencies to channel 22.  Since the duplex system is not in common use, however, an engineer is more likely to say "let's go to channel 22".

The radios have buttons/knobs to actually "dial in" both the TX/RX channels and are displayed on the front of the radio.  These channels are based on the American Association of Railroads' (AAR) breakdown of the 159.810-161.565 Mhz frequency range allocated by the FCC for railroad voice communications.  Here's a link to a table relating frequencies to AAR channel numbers:

http://on-track-on-line.com/scanner-radio.shtml

Generally speaking, channels 7-97 are the main ones to focus on.

I always setup my scanners with all the AAR channels because you never know when things will change, when you might take a trip to a new area with different frequencies, or when crews might change channels for a little informal chatting.  Soo Line crews operating on CSX's ex-C&EI line would occassionally tell each other to switch to channel 44 (ex-MILW frequency) to discuss where they stood in the crew pool.  If you don't know what channel 44 is, then you're going to miss out.

The other reason to key in all the AAR frequencies is when the ionospheric "skip" kicks in and you can listen to railroad communications not normally within the range of your antenna.  FM radio transmisisons are usually line-of-sight.  However, the ionosphere, especially on clear summer mornings, can turn into a "mirror" that reflects FM transmissions back to the ground sometimes hundreds of miles away from their original source.  This is also referred to as "tropospheric ducting"  During these periods you can unlock all the channels and hear all sorts of interesting things.