18"/72' baggage/combo is correct for the prototype (aka it's scale). The rest of the streamline cars were 85' long and should be represented by 21" cars.
If you go with the 72' cars for everything else a 15"/60 foot car would at least be "proportional".
Chuck
Very good point. Don't Lionel and MTH make most of their 18" passenger sets with baggage cars that are also 18". Wonder which way most people would prefer or consider "more correct"
John