Others have already made some good points.

First, there is a difference between remote base stations and
repeaters. A line may have one or the other or both.

Remote base stations are what trains use to talk to dispatchers,
in most cases.

Remote base stations are just what the name implies. They are
locations from which the dispatcher can talk to radio units in
the field, which can include engines, MoW equipment, vehicles,
and even hand-held radios. These remote base stations are
connected to the dispatch center by land line, microwave, or
some other means.

As others have noted, dispatchers cannot monitor all the remote
base stations in their territory. Instead, they have a display
that indicates that someone has toned a particular remote base
station, at which point they then select that base station for
further communications.


Toning can take several forms and systems can vary from railroad
to railroad or within areas of the same railroad. All remote
base stations in an area can be keyed to the same trigger tones.
In that case, the radio system selects the base station with the
strongest signal to display to the dispatcher when a field unit
tones a request to connect.

In other cases, each base station has its own trigger code, and
the crew in the field has to select which base station it wants
to communicate through.


Most systems also have an emergency tone code, such as toning 911,
instead of the normal code. That indicates to the dispatcher that
a particular remote base station has a pending emergency -- and
the dispatcher will answer that call first. That can be very
important for a busy district where the dispatcher may have lots
of pending communications.

I hope that helps.

-- Ernest