There are several cabooses in the lower 90x numbering range based at Naval Support Activity Mechanicsburg, PA. The caboose you saw is one of them. They come in various color schemes and inside you'll find radios, food preparation equipment, and sleeping accommodations. I don't think MPs ride in them, but instead they carry DOD guards. Given the other types of cars at NSA Mechanicsburg (visible from the fence, too), it's a fair bet the cabooses are used as escort cars for nuclear loads. (Before anyone gets bent out of shape at the mention of this information, let me add this is obvious just from casual observation, and even if someone doesn't notice then surely one's seeing dog does.)