I have the Illinois Central E6 ABA units with the aluminum passenger car set. The diesels are better looking than my MTH IC Premier units, but the plastic MTH cars seem to look a bit more realistic that the K Line aluminums.
Also have the Chicago and Alton Heavyweight Passenger car set and recently purchased one of the newer Lionel 18" C&A coach. You can barely tell the difference between the the Lionel and K Line C&A passenger cars which, according to a modeler friend, is due to the fact that Lionel used the K Line molds for its recent production of heavyweights.
I just recently picked up almost all the Chesapeake and Ohio K Line heavyweights and am in the process of trying to remove the silhouettes from two of the cars and replace with K line interiors. Picked up a K Line heavyweight ACL diner car (Valdosta) with the intention of cannibalizing the plastic interior for the C&A Swan Tavern car, but found that the ACL Diner was just too striking to disturb its appearance. The Valdosta will now serve as a "loaned" diner on my MTH Illinois Central Premier City of Miami.
Guess i will have to look for interiors for the silhouetted K Line C&O Swan Tavern and the C&O Ft. Eustes pullman car at York.
I have been searching (without any luck) for a few Gulf Mobile and Ohio MTH Premier passenger cars to integrate into the K Line Chicago and Alton consist - the goal being to reflect the 1946 takeover of the C&A by the GM&O by running a passenger train with both cars present.