K-line did indeed make a 2 pack of matching 18 inch cars for the Spirit of St Louis. I don’t have the car numbers handy but one was an RPO and the other a Pullman from what I remember.
I’ve seen the 4pack of 18 inch cars in both the yellow/ black and black boxes. Have only seen the 18 inch add ons in the yellow/black boxes.
I’d you’re looking for an extra long train, you can add the 18inch business car. It will couple no problem with the flat end observation.