When it comes to modelling the lights employed on railroad locomotives, it takes a bit of good research.
First; what country's railroad(s) are you modelling ?
Second, what era are you modelling ?
Modern railroading, generally meaning "Today" or within the last 20 years; has seen many changes in the use of lights, as part of the operating rules, in North America...of which, I only have "Some" knowledge of...
Today, "Classification lights" are NOT generally used...if ever. (Most new locomotives don't come equipped with them) They are/were the small lights on the front of the locomotive. They were generally either "White" to indicate an extra train, not on the timetable; or "Green" to indicate a following section, of a train on the timetable. Red only was used if the locomotive was operating in reverse, making it the rear of the train, and then the red lights were considered as "Markers", or marker lights.
It is generally a rule that head lights, and "Ditch lights" are employed on the leading locomotive in a consist (Train). This is for safety; warning of the train approaching. If other locomotives in the consist have their headlights on, it's because the crew forgot to turn them off... were just plain lazy....or didn't give a durn !!
The lights are controlled by the person operating the locomotive, and are not generally applied automatically.
If a crew member is lazy, or just operating within the yard limits (Switching)...you might see anything...if there are no supervisors around....!!
Today; "Marker lights" are seldom if ever mandatory. The old rule was that a consist had to have "Markers" bringing up the rear, in order to be a (Complete) train. "Marker lights" were never on the "Front" of a train.
They have been replaced by "Fred" a (Sometimes) Flashing, Rear, End, Detector, which replaces the caboose, and marker lights. A Fred monitors the air brake pressure, and is connected by radio to the cab of the leading locomotive. A Fred, is basically a box, which is connected to the air line of the last car of a train, sometimes equipped with a flashing light. It is mounted on the coupler, and is placed or removed by a crew member. It is NOT permanently mounted on the coupler, and is battery powered.
Cab lights are NOT turned on at night, as they would ruin the operator's night vision. Small desk lamps provide light for reading train orders. Other lights, mounted on equipment, are governed by the railroad which owns the equipment...such as beacons, etc
I hope this helps a bit, for those interested...."Experts" are welcome to make corrections, or add more information.
Fr.Fred