FREDs have almost completely replaced cabooses (cabeese?). The idea behind them is to provide a means of dumping the air from the rear end (in addition to the engineer dumping the air from the loco), to provide an illuminated marker at night, and to provide a means of transmitting the air pressure reading from the rear end to the head end. Another function, rarely mentioned, is the ability to inform the engineer if the rear of the train is moving or stopped. This last function is valuable to let the engineer know when he can safely apply more power when starting the train. All of the functions were formerly performed by the rear end crew.
FRA rules define when the ability to dump the air from the rear end is required. It takes into account both tonnage and maximum length of grade and percentage of grade on the route transversed by the train. If the tonnage, percentage and length of grade is not exceeded, a FRED maybe replaced by a simple HVM (high visibility marker) that is actually just a red marker light. For operation during daylight hours, even that is not required. A red flag is sufficient.
The purpose of the red flag/light is to provide a positive indication to an observer that the car displaying it is, indeed the last car of the train. By knowing that, an observer would know that the rear portion of the train was not left on the track somewhere. It also tells them that the last car has passed a given spot. It is important for the engineer to know that, as when clearing a speed restriction, yard limit, or milepost location. The last is important when operating in Track Warrant Territory, as the dispatcher can then assign the track segment just cleared to a following train. The engineer can know the location of the rear of his train without a rear-end crew by measuring his train using the distance counter on the locomotive.
A train may reverse direction on a main line with dispatcher's permission, or if occupying the main line on a "work between" track warrant. But a crew member should be protecting the movement. This includes getting off and flagging road crossings. If the locomotive has "all lights on" it does not satisfy the requirement for headlights at road crossings if it is on the opposite end of the train.
Les