452.925   DPU (3/7)
452.9375  Head End EOT <<    
452.950   DPU (2/6)

457.925   DPU (1/5/9)
457.9375  Rear EOT     >>    
457.950   DPU (0/4/8)

These frequencies carry digital signals, which are heard as beeps on a regular radio. While the data in these transmissions requires computer software to decode, hearing the beeps can be very useful in determining if a train is in the area.

According to the railroads, on the mainline, a 'train' consists of three things: a locomotive, a marker identifying the rear end, and movement authority (permission to be where it is). Without all 3, it is not a complete train. An EOT (End of Train Device) is a marker that uses air pressure from the rear end of the brake line to power a flashing red light and a computer system that transmits air line pressure and GPS position to the locomotive. This lets the crew know the location and braking force of the rear of the train. EOTs transmit data every minute or so, and the locomotive responds with a similar sounding chirp about as often.

DPU stands for Distributed Power Unit – a locomotive placed in the middle or at the rear of the train and controlled remotely from the front. The DPU frequencies listed are how data is sent from the lead locomotive to the DPU(s) to tell them what to do. The numbers are the last digit of the lead locomotive, though depending on the railroad this convention holds true about 90% of the time. Similar to an EOT, the lead locomotive regularly sends updates to the DPUs. These transmissions are very short; a scanner scanning a large number of frequencies might miss the chirp altogether if it happens while other frequencies are being scanned. When commands are sent to the DPU, the transmissions are longer; a train throttling up from a stop makes a continuous series of long chirps that are hard to miss.

From these simple beeps, we can deduce if a train is in the area, if it's facing towards or away (front vs rear EOT signal strength), if it's moving towards or away or stopped (signal strength over time), if it's long enough to have a DPU, if a long train is adjusting speed or not, and make a good guess as to the last digit of the engine number. Note that passenger trains don't have EOTs or DPUs and this becomes more useful the longer the train is.