Although many people lump the two together, there are actually two different types of radio communication stations you will find out in the field: repeaters and remote base stations.

As the previous poster indicated, repeaters simply retransmit all traffic received on one frequency on another frequency, and, usually being located on high ground, make messages available over a wider area. You will find repeaters in mountain terrain, where mountains would normally block the propagation of normal signals. Repeaters are also used in long tunnels. Many of these long tunnels have a continuous antenna running the length of the tunnel, tied in with a repeater outside the tunnel, which also has its own external antenna. This makes it possible for trains inside to the tunnel to receive and transmit radio messages -- and for trains and other personnel outside the tunnel to receive these messages.

Remote base stations are the stations in the field that a dispatcher, located many miles away, can use to talk to trains and other railroad workers. These are tied to the dispatch center either by land line (regular phone circuits, which can include fiber optic cable) or by microwave links. You may als ofind remote base stations that are tied in to the railroad's PBX network (internal telephone system). Trhough these, any railroad employee with a radio equipped with a touch-tone pad can dial into the telephone system and connect to any internal or external number as needed.

Either type of these transmitters can be in any type of metal equipment housing, similar to those used for other signal equipment circuitry. Where the railroad already has a building at the location (or large equipment housing), the remote base station equipment could be located in the building. In most cases, remote base stations have a fairly tall antenna tower. 70 feet is one of the more common heights, as it helps cover a fairly substantial area without being excessively expensive. The actual antenna(s) mounted on the tower can be either directional or omnidirectional. For example, to get the most coverage for an east-west line, you would want a directional antenna aligned with the direction of the track. In a yard or other area where there's activity over a wide area, an omnidirectional antenna would work best.

In a few cases, where there are existing tall structures (buildings or antenna towers) available, the railroad may pay the owner of those structures to mount their antennas on them. In yards with tall lighting towers, you may also find antennas mounted on thse.

-- Ernest