Technically a 'unit train' is a train of cars that are treated like one car. An example would be cars for hauling coal, where a train is loaded in the Powder River basin, run as a unit to a midwest power plant, unloaded going around a loop, and run back empty - all without being uncoupled or switched.
Refrigerator cars were never used in that type of service. If you lived near say the Santa Fe mainline long ago, you would see many trains that were all reefers, but the cars in the train would be different every time. Otherwise, these cars were just regular freight cars. They could be moved to different places a few at a time to load, and be separated at their destination to be unloaded. Seeing one or two in a train by themselves would be very normal.
