I may have missed what Ricky was saying. If you have K-Line cars and want to add the platform off a Lionel car the dimensions are off in height and width. Lionel 18" cars are the same dimensions as their 21" cars though.
K-Line on the left, Lionel right.
FYI UP made the buisness cars out of other cars without the platforms. It may actually be easier to do likewise.
Pete