A reefer is also a
refrigerated boxcar, generally used for express shipments of fruit and vegetables. Most reefer cars have slogans such as "Solid Cold" stenciled on their sides, to indicate their special status among
railroads' rolling stock.
Reefers have noisy cooling fans running at all times, and are not seen very often, except on the specific lines where Tropicana and others operate daily juice trains. They're also pretty damn cold to the touch, oddly enough.
Origin of the name comes from the "reef" in "refrigerator", I'm guessing...