Some railroad frequencies can be assigned to a non-railroad user in areas where the frequency is not in use; that particular frequency (AAR 005) is only used by railroad trucking services in the US. Chester County has been using that one as long as I can remember; their usage may predate the creation of the AAR standard channel plan.Wow...this morning I'm getting Chester County, PA's fire/ems dispatch on 160.1850 here in Jefferson County, WV. Interestingly enough, I thought that frequency was allocated to the AAR.
The Chester Co frequency actually is 160.185.The fire department I was with a few years ago had something similar. We used Minitor 5 pagers in Hoke County NC and every once in a while we would pick up the signal from Myrtle Beach SC. Have you checked to see how close the Chester County PA fire/ems dispatch frequency is to the 160.1850 frequency you monitor? I bring it up because when it happened to us I was curious as to how close the frequencies for Hoke County NC and Myrtle Beach were and it turned out they were pretty close and the atmospheric conditions must have been just right for the signal to travel.