Marker lights and classification lights.

A recent forum question asked about the use of colored lights
on the front of locomotives, what they were for, and if they were 
considered to be markers.

Usage varied over time, and from railroad to railroad, but there are some
general practices that have been followed.  To be sure of use at any
particular time or place, try to pick up a railroad rulebook for your 
favorite road for the time period which interestes you.  Usually, markers
and classification signals were shown in the rulebook.  Sometimes,
this information is put in the Employees Timetable rather than the rulebook,
so you may have to get one of  these too.  Flea markets are often a good
source, or check TrainNet Swap Forum.

Generally, "markers" refers to the signals displayed at the back end
of the train.  In fact, the usual definition of a train is an engine,
or more than one engine coupled, with or without cars, displaying
markers.  Within the last few years, the definition has usually been
added that it also includes authority to occupy the main track.

The purpose of markers is to give positive indication of the rear end
of the train.  If, for example, a meet was scheduled between two trains,
without radio communication, seeing the markers of the opposing move
gives proof that the train is complete - that no part of the train has
been left behind.  If you meet another train, but do not see its markers
you cannot leave the station without confirmation that the whole train
has passed.

Traditional markers displayed green (some, like the ATSF, used yellow,
but that's another story <g>.  See below.) to the front and sides, and red to the
rear when on the main track.  When clear of the main track, the markers
were turned so the red lenses faced each other, with green to the rear.
When running against the current of traffic on double track, one marker
showed green to rear and one showed red to rear.

Other markers, such as flags or reflectorized paddles were often authorized.

Today, a red flag or light, or flashing red light is commonly used, and
a "highly visible marker", also known as End of Train (EOT), Rear End
Device ("FRED" or "Freddie" - you can guess what the "F" represents
in the typical railroad vocabulary!) or Telemetry device is usually used.

Classification signals were used on the front of the train.

A Regular train, that was authorized by the Employees Timetable and had
a schedule, normally carried no class signals.  If permanently mounted, they
were not illuminated if not needed.

Extra trains, that is trains that were authorized by a Train Order issued
by the dispatcher, showed white class signals - lights by night, lights
and/or flags by day.  These were displayed at the front of the engine.
Extra trains crews studied the schedules of regular trains very carefully,
and could proceed only when they were certain that could make it into the
clear at the next siding at least 5 minutes before the scheduled arrival
time of any opposing regular train, or 10 minutes ahead of a faster moving
train in the same direction.

Thus, meets between a regular train and an extra took place whereever
the extra crews happened to be.  The regular trains crew often would never
even know about the meet until they saw the extra in the siding, since it 
was the extras responsibility to stay out of the way of the regular trains!

Meets between opposing extras could only be made when arranged by the
dispatcher by Train Order.

The white class signals clearly identified the extra as an extra, so it couldn't
 be mistaken for another regular train that might also have been scheuled for
about the same time.

When traffic was heavy, sometimes regular trains became too 
long to run as a single train, so it was broken into two or 
more Sections, spaced anywhere from a few minutes to several 
hours apart.  All sections, except the last, carried green 
class signals to indicate that it was being followed by another 
section of the same train with the same schedule authority.  
Opposing crews seeing green on the front of another train could 
not regard that train as complete until they saw the rear 
markers of another section that was not wearing green.

Today, most diesels have their class lamps welded over.  Train 
Orders are a thing of the past, and today  trains move as 
extras of equal class.  The dispatcher confers authority for 
movement through Track Warrant Control (TWC territory), Direct 
Traffic Control (DTC territory), or Centralized Traffic Control 
(CTC territory) using radios and/or trackside signals to 
communicate.

Regarding use of the color green:  In the 19th century, people
understood red to symbolize danger, just as today.  Caution,
however, was signified by green!  This wasn't unique to railroads,
as I have an old architecure book that warns builders not to leave
open excavations without putting up green warning markers,
"green being the universal color for caution."

Railroads adopted the use of a white light to indicate "Proceed",
and since the lense for this was clear a proceed signal came to
be called "clear".  I believe there's a line in "The Ballad of Casey
Jones" to the effect "Fireman says to Casey 'You've got a white
eye'", which would indicate a clear signal in the language of the
day.  

My 1917 Milwaukee Road rulebook shows semaphore signals
using red, green, and white.

In 1918, the American Railroad Association recommended 
railroads switch to using green for a proceed indication, and
use yellow for caution, since under the old scheme a broken
or vandalized red or green signal would falsely give a less
restrictive indication - namely, clear!  A few railroads changed
their markers too, resulting in red/yellow markers on those roads.

Interestingly, to this day rules identify a green signal with the 
name "clear", even though the indication may not necessarily
be that the track ahead is actually clear.  For example, a 
green signal with the letter "D" on a plate below the signal
light is called "Distant Signal Clear", though the signal conveys
to the engineer no information about the condition or occupancy
of the track immediately ahead.

I hope this information helps people understand these signals, 
makes their railfanning more interesting, and their modelling 
more fun.

-JLS