Here's a few of my box cars............
All 3 of these are Lionel Standard "O". Back in the 1970's, Lionel started producing some Standard "O" freight cars, which were actually 1/48th scale, as opposed to their smaller "Traditional" and smaller yet "O27" offerings. These Standard "O" freight cars fit in a lot better with
most of Lionel's postwar diesels, which also were 1/48th scale (or pretty darned close to it).
The two Rock Island cars were stripped, repainted and decaled with K4 decals. The GTW car is simply a stock Lionel car. In all cases, I lower the car ride height for better looks (a lot of O-gauge cars from ALL manufactures tend to ride too high for my tastes). I also drill new holes in the floors, so the trucks can be mounted with the screws at the truck bolsters, instead of that other die-cast protrusion on the trucks. Unfortunately, this body lowering and truck refitting would probably be out of the realm of most modelers, so I won't get into the specifics here. I add stirrups at both ends of the cars made from Arrow staples (the ones used in Arrow staple guns). Where necessary, I'll paint the door guides and the roof walks to match the color of the car.
Speaking of roof walks, on some Standard "O" box cars (probably later production ones), for whatever reason Lionel made some changes to the body roof walk mounting protrusions and the roof walks themselves sit too high above the roof tops. So I cut/file/sand those protrusions down to size in order to get the walks down closer to the roofs where they belong.
Why would anyone go to this trouble, you ask? If you shop around, both online or train shows, you can sometime find these older Standard "O" cars for as little as $10~$15 or so. And even though they're pretty nice as is, the looks can be improved without a whole lot of trouble or expense, and you end up with even nicer looking freight cars yet, at a fraction of the cost of newer offerings from the manufacturers!