Train Reports

That is one of those automatic Hot Box, or Dragging Equipment detectors. They are automatically activated when a train passes its location, and after the complete train has passed, it radios its report. Some detectors also announce when you "trip it", i.e. when it begins "inspecting" your train, then gives its final report after your train has completely passed. Also, some detectors provide train speed, ambient temterature, and number of axles.  

On the A-line CSX calls them "Defect Detectors" and as Hot Water said they usually announce the devices location by mile marker, train direction, total # axles and any defects.

Who gets the report from the detector?

It's automatically broadcast by radio so it's heard by train crews and dispatcher.

Some railroads have dragging equipment detectors every 10 miles or so. It used to be that sometimes just one dragging wheelset would cause a huge pile-up when the train came to a switch or crossing. If they get an early notification of dragging wheels or other anomoly, the train can be stopped and inspected before the problem escalates.

Overheated wheel bearings can also be detected by trackside sensors. A steam-powered special train will typically trip the detector because of the heat from the firebox!

One time I was riding Amtrak out of LA and the conductor was walking up the aisle, and his radio was squawking with a trackside detector report which gave an axle count that was not divisible by 4. And I thought, what car or loco on this train does not have 4 axles? because it would be sometime non-standard.

At the next station stop I walked the platform and saw private car "Virginia City" on the end of the train, returning from a meet of private railcars. Virginia City is an old six-axle heavyweight car formerly owned by Lucius Beebe.