The Atlas cars are exceptional, but very hard to find. Another higher-end short car train would be the GGD SP Harriman sets. They are also 60' cars and came in five car sets. They are pricey but accurate to the 60-C-5 style cars that SP operated lots of.
However, for a more budget conscience approach, the MTH Railking Cars with the 30- number would be a nice starting point. Lionel's "Madison" style cars that are based on the post-war cars are about the same size as are the Williams 15" cars. The Williams cars have window inserts, but you will get the best price on those likely. I keep a set of CNJ ones for times I operate on public layouts with 027 or 031 curves.
Just a few photos of the Williams 15" cars. These came in lots of western road names also.