White: Indicates an extra train. This is still used in Canada on any subdivision that is NOT CTC, and has scheduled train service. (I can't think of any, but it is possible.) If the front of the lead unit does not have marker lights or flags, the trailing end of the lead unit must have the lights turned on.
Red: Used as a rear of train marker. Examples of this are light engine moves, pushers and perhaps snow plough trains if there is no caboose trailing. Power on yard or transfer assignments must have the rear headlight on dim. This can replace the red rear markers otherwise required.
Green: Not required by rules anymore in Canada (both white and red still are) - indicates second section of train to follow. When areas were governed by timetable authority, green markers would indicate that there is a second section of the train to follow, and it can follow up to 24 hours behind the first section. If the second section also displays green marker lights, that indicates there is a thirde section to follow, etc.
Then there were also marker lights on cabooses:
On single track, and when running with the current of traffic on double track: red to the rear.
On double track running against the current of traffic: red to the rear on the outside and green to the rear between the tracks.
On multiple track (against or with the current of traffic) red to the rear unless otherwise instructed by special instructions.
When a train clears the main track (e.g. goes into a siding) permitting another train to pass, green markers must be shown to the rear.
If a train has a flashing red marker at the rear, it must be switched to a flashing green marker in the above instance.
If a caboose is not equiped with a green signal to indicate it has cleared he main track, the red light must be replaced with a white light to indicate the train is clear.
All of these caboose rules were contained in the UCOR in Canada, which was replaced by the CROR in 1990. The only rules in the CROR are the first two (white and red markers on engines). ALso in the UCOR, engine markers were to be displayed on ALL engines on a train.
----------------------------------------------------------------- Gordon Webster Internet: email@example.com Contributing Editor CIS: 72122,3353 Rail & Transit