The class lights or flags (I believe the rule book term was signals) on the engine helped identify the train for movement authority purposes. The simplest example is if an engine had green signals an opposing train knew that he had to keep out of the way of following sections of trains on that schedule.
Marker lights "marked" the end of the train. This worked in several ways. It helped following trains "see" the end of a train ahead. If a train went into a siding, the markers were turned from red to green (or yellow) once the rear of the train was clear of the main track indicating it was safe for the other train to pass. If you passed a train and the last car went by without markers, then it meant the train was not fully by you.....for example perhaps it was doubling a hill and had left the rear part of the train further down the track.
This has already gotten longer winded than I intended, but class lights and marker lights were part of a complex system of dispatching designed to be as fail safe as possible when there was very little communications along the line. No radio, and maybe just a single telegraph wire between distant stations.
Train dispatching (both at the dispatching and train operation end) back in those days was a fascinating and complex skill, and mistakes could be deadly. Books were written on the subject ("Rights of Trains" comes to mind).
Two final comments. Rules did vary between railroads, and in some cases were simplified. For example I believe the DRGW eliminated the use signals on the ng. in the final years because everything was extra. Most engines still had the class lights, but they had no significance.
I wrote the above quickly, so if I got anything wrong, corrections are welcome.
JBWX
Edited 1 time(s). Last edit at 06/10/2009 04:29PM by John West.
