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Historical Society of 

Mount Airy, Maryland, Inc.

B&O Railroad Color Position Light Signal

To minimize the chance of collisions, railroads used signals to advise train engineers and conductors about track conditions ahead.  The Baltimore & Ohio Railroad’s Color Position Light, or C.P.L.,  was one of the most unique signals in American railroading.  They used lights to mimic the arm of a semaphore flag signal (shown below).  

Semaphore Flag Signal

The B&O C.P.L. used green, amber, lunar white, and red lensed lights along with position to give command signals.  It used two lamps per color/position  in case one light failed.  The B&O also used modifier lights or markers, of white and amber to provide additional signal combinations.  The B&O’s original C.P.L. system had 16 different signal combinations called aspects.  This signal from the Mount Airy Junction, or East Plane, is preserved in its final configuration, which had two  aspects as explained below. 

CPL Signal

The C.P.L like most signal systems used batteries for power.  The signals would normally stay dark (off) until a train approached.  This would help conserve battery power.    The signal at Mount Airy Junction was taken out of service by CSX in 2004.   For more information about B&O Railroad CPLs  click here.    

Green with Marker

Track ahead Clear

Proceed at Current Speed.


Track ahead Clear

Proceed at Slow Speed.

Green Flashing

Track ahead Clear

Proceed at Slow Speed through switches and turnouts, then proceed at Current Speed.

Red with Marker

Proceed at Reduced Speed.



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