Lionel made some short KCS cars back in the 90's. They were aluminum, had very nice graphics, and are not real easy to find now. They also made a set of F units to go with them. I think the cars were 13" but they might have been 15". You can find them on eBay but they go for a pretty good price.
I have a couple of sets of MTH KCS cars, one set of which is for sale. These are 18" ABS (not aluminum) cars. As far as I know MTH never made KCS cars in aluminum, and I've never seen anything in KCS from Weaver or anybody else.