These signals are used for motor-car protection.  Because motor-cars are relatively lightweight compared to trains, a motor-car does not maintain a connection to the track adequate to reliably shunt the track circuit.  Some motor cars also had insulated wheels.  Therefore, a motor-car entering a block is not able to provide itself protection through the normal track circuits designed for trains.

To help alleviate this problem, a small device called a “motor-car indicator” is mounted on a short pole near the track to provide the motor-car crew with information as to whether a train occupies a particular section of track.  This section, can contain one or more blocks of track and usually includes the section to the rear of the indicator since approaching trains are of the greatest interest to the motor-car crew.  Often, these signals have a push button which enables the crew to check a second section of track by pressing the button.  This is helpful if trains travel in both directions on a single track or multiple tracks are involved.

Origin of the Track Circuit

Right.  Signal indicating section unoccupied.

Below.  Motor-car signal with pushbutton located on the front near the bottom of the signal case.

Below.  Front and back views of another type of motor car signal.  This could have been located in a relay box.  Vertical dots indicate clear.  Horizontal dots indicate occupied.