It should be noted that railroad signals should not be confused as wayside railroad signs, they perform a completely different function. Railroad signals
are an integral part of railroading, without them there is absolutely
no way trains could safely operate, particularly on single track lines.
However, more than just for safety signals allow for the efficient and steady flow of trains, enabling several to operate over a single-tracked line. Signals have their beginnings dating back to the industry's infancy itself with the famed ball-type signal.
Perhaps, though, the most famous signal-type of all was the
semaphore, which came into use in the mid-19th century and even today
can still be found in regular service!
While signal displays and meanings often varied from railroad to railroad some of the most common types were the color-light signals
(or tri-lights), searchlights, position-lights, and color-position
lights with the tri-light the most popular today. The first railroad signals
came into use in 1832 on the New Castle & Frenchtown Railroad but
really came of age in 1872 with the invention of the track circuit by
William Robinson. This circuitry allowed for the steel rail's natural
conductivity to be employed thus giving birth to automatic block signaling systems, the leader of which remains Union Switch & Signal created by George Westinghouse in 1881. Today US&S is part of Ansaldo STS, an Italian company.
One of the less recognized wayside railroad signs are the quarter, half and three-quarter mileposts. Before the days of GPS and computer-controlled signaling systems these mileposts helped train crews gauge their location between each mile marker not only for purposes of knowing how close the train was to its destination but also in the event of an accident or derailment. Different railroads used different indications on these "less-than" mileposts. Some simply used fractional numbers (i.e., 1/4, 1/2, 3/4, etc.) while others like the Southern Railway used a series of dots and dashes to denote the meaning. In any event, all the signs listed here are certainly not every type that was used just a cluster of many of the best recognized.
If you are interested in fallen flags and classic systems before the modern merger movement took hold I would suggest purchasing one, or all three volumes of Mike Schafer’s Classic American Railroads series of books. The publications feature a wealth of information and are loaded with coloredphotographs covering names like the Baltimore & Ohio, Santa Fe, Pennsylvania, Milwaukee Road, and lots more. If you have any interest in most famous bygone railroads which once operated in the U.S. I would strongly recommend picking up one, two, or even all three of Mr. Schafer’s book. I truly cannot say enough just how enjoyable and fun they are to read and peruse through.