The number boards of the engine that identifies the train will be lighted. All others will be dark. Examples: "Extra 2929 East; Engine 5517 West; No. 18, engine 20) This is so that other trains or engines which have been given authority to occupy the main track after the arrival of another specific train at their location, can verify that the train they are waiting for has passed their location.
Usually, this means that the leading unit has number lights illuminated and others are dark. There are exceptions, but only rarely.
Ground lights running board lights, and step lights (controlled by the same switch on the control stand) are supposed to be lighted on the leading locomotive unit, and dark on the others.
