The Lionel GS2 from the 90's may be a good candidate for those cars as it proportionately correct, but just smaller than than full 1:48 one. It is similar in scale to the postwar J. If you can live with a conventional locomotive, it is a decent option. The colors won't match if that is important to you. K-line's colors are way off. I have 18 of these in the 21" size and they are just too dark. My locomotives don't match color wise. That doesn't bother me though.
Another option might be the Lionel Daylight painted Atlantic. It is 1:48 proportion, but as a 4-4-2 it is shorter and may look better with the 15" cars.
Finally RK E8s and PAs might work for you as well if you can live with a diesel.
Good luck!