Right on. Refrigeration units don't "make cold." They remove heat. As long as they remove heat from a space faster than the heat leaks back in from the outside, the space gets colder. Once you reach the point where the heat leaks back in faster than the unit can remove it, the space stops getting any colder. With some types of produce you also have to factor in heat of respiration as well. Citrus fruit produces gases that make heat and cause them to be harder to cool down than other fruits. That gas is the strong "orange" smell you will get if you open up a trailer that is loaded with oranges. As an example of the difference between the insulation of a rail car and a trailer, the doors on the back of a trailer weigh a few hundred pounds. A plug door on a refrigerated car weighs about 2000 lbs if I remember correctly, and the mechanism that closes it makes a much tighter seal than any trailer door made.
Josh, the units are not way oversized for a 53 foot trailer. The other problem is that trailers are not constructed as well as a rail car, and the reefer manufacturer needs to make the reefer with enough capacity to hold temps even when the trailer suffers damage. Rail cars are made with thick steel walls, trailers are made with thin aluminum skins. If a plug door sticks, the workers use a forklift to push it open or closed. If the forks hit the side of the car, they scratch the paint and maybe put a dent in the side. If a fork lift is used to push a tight door closed on a trailer and it slips, it punches a hole in the trailer. A hole in the trailer is a heat leak. The life expectancy of a trailer is probably less than 10 years. I don't know what Kroger's practice is, but before Kroger bought out Ralph's markets, Ralph's practice was to keep a trailer only 9 years before replacing it. A rail car is expected to last 40 years barring wrecks.
