Dixie Union Station is on my list... possibly Roundhouse if we get to Louisville. Anything else?
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Just an FYI, I am from Louisville and live in Cincinnati. Roundhouse expanded a few years ago, Didn't know that the building had so many hallways . You will have a larger selection of Used product, (O gauge) if that is what you are looking for. Nothing against Dixie Union...
Best place for postwar trains in Chicagoland has to be the monthly train show at the DuPage county fairgrounds, next show is the morning of Sunday May 6, 2018.
http://greatmidwesttrainshow.com/
Highly personal opinion, if you are driving near Indianapolis drop by Mr. Muffin's trains in Atlanta. Note that there are two locations, a store and a layout. I don't recall any postwar but you still need to see his (their???) store.
If you're in the Chicago area Berwyn's Toy Trains on Ogden Ave (old US 66) in Berwyn, IL has a good selection of excellent postwar Lionel Trains, many with original boxes.
Berwyn's Trains, Berwyn, IL and America's Best Hobbies, Itasca, IL, both usually have a good amount of PW items. Phone first. I second the recommendation regarding the Great Midwest Train Show at the DuPage County Fairgrounds in Wheaton, IL, monthly except July.