White flags on the smokebox, running forward by day, means the train is an "extra," and not on the timetable, like a freight train. White class lamps at night, along with the flags, meant the same thing.
Green flags on the smokebox, engine running forward by day, means that there is a following "section" of the same train. Green class lamps at night, along with the flags, meant the same thing.
This is a simplification. There were other combinations of white and green flags, either smokebox or pilot beam mounted, that meant different things, depending if the engine was running forward or backward.