I have used MTH tires as replacements for Lionels and K-Lines, plus others, when the original spares that came with the engine are gone. The ones that mostly get used are DE-25, DE-27, and DE-31 in my experience. (Note these numbers don't include the insane series of five or so useless zeros that MTH puts in all it's part PN's). I sure hope they will still be available after MTH is defunct.
Rod