You may want to ask this question in the FLA state forum, or the Railroad forum. I know in GA that CSX and NS use dispatch to train and train to dispatch frequencies. It most likely is the same where you live.I was trying to see if it could be used on a railroad radio system were they talk on one frequency then receive on another. I not sure if it can be used that way or if it's intended to be used for repeater input and output situations.
Since the R20 has dual VFO's, why not program the two frequencies in both VFO's, VFO A has the Dispatch to Train, and VFO B has Train to Dispatch?I tried using the duplex function with 2 railroad frequencies. Dispatch-Train and Train-dispatch and the offset value wasn't available in the R20 so it won't work in that situation.
What are the two frequencies? I just downloaded a manual from Icom America and it showed that there can be a value between 0.0000 and 159.9999Mhz. This means that you can set the offset between 0.0MHz and 159.9999MHz. Just subtract the lower frequency to get the difference. Then use that difference as the offset. Then the -DUP/+DUP is set with regard to the Disp-2-Train.I was just expereimenting to see how the Duplex operation would perform in the R20 using those frequencies.The offset value that would be needed isn't selectable.I normally monitor those channels the way you just desribed.