Most of the K-Line scale boxcars you will encounter are numbered in the K761-xxxx series. These are molded plastic boxcars of the correct scale size for a PS-1 car, but not up to the standard of the much newer Lionel PS-1 car. Here's a picture:
The biggest problems with these cars, from a scale point of view, is that they have old-fashioned stamped metal door guides and the stirrups are part of the stamped-metal chassis. They look OK in operation but they were made 6-12 years ago and are nowhere near up to newer cars in appearance.
I don't know if the die-cast cars are scale or not. I don't have a high opinion of the detailing on them.
K-Line did market some freight cars made by Intermountain, or assembled out of Intermountain parts. This was an early K-Line venture into scale and they only made a few. All the ones I've seen are reefers and, as mentioned by others, they are extremely fragile. These reefers were numbered K-76xx. I haven't personally seen a K-Line/Intermountain boxcar, but they may exist. Here's a picture of one of the steel reefers.