Radio repeaters are usually located on the top of a mountain, hill or another high place and they are intended to expand the range of radio communications by repeating everything on a certain frequency. Open repeaters repeat everything on a certain frequency as it happens. Radios can tune in by changing to the repeater frequency to listen, while transmitting on the road channel.
BNSF has added repeaters to two of its yards in the "FUNNEL" (Hauser Yard and Yardley Yard) as cuts of cars being switched have become longer than walkie talkie range, repeaters have been added to ensure reliable communication between all parties switching in the yard. (BNSF is doing this to more and more yards all the time). Signal and communications (phone) repeaters can be trackside, or in a high area depending on need in that area, and certain types of signal communications can be seen (track diagrams, signal indications) using a scanner modified to hook into a laptop computer. The boxes can be anything from a signal shack with a tower next to it, or a phone pole with a box bolted to it and an antenna bolted to the top. Directional repeaters may have a focused antenna resembling a "dish network" type dish as well. Railroads also use microwave systems to transmit communications over a large area, but in a focused beam and at a frequency that no scanner can pick up.