I have a Lionel #731 Freight set from the fifties that I believe is O-27 Gauge and I bought a K-Line ,Phoebe Snow, Wood Sided Refer Box Car to add to the set which has O-Gauge on the box and it seems a bit smaller than the cars in the #731 Set, especially the original New Haven Box car in the Lionel #731 Set.
John
The New Haven box car in your No. 731 set should be a 6468-25. It is a "traditional" O gauge or O-27 gauge car that is not a scale model of any particular real world freight car.
If your K-line reefer is a K642 it would be shorter then the Lionel 6468 box car. A K-line 742 reefer would be larger the the K642 and about the same size as a Lionel 6464 or 6468 box car.
Lionel, MTH and Atlas all have O and O-27 sized toy train product lines. And they all have O scale product lines.
Lionel calls its toy trains "traditional." Since the 1970s Lionel's scale products are marketed as "Standard O." Some of them, like the Lionel PS-1 box car, are well regarded by 2 and 3 rail O scale modelers as very dimensionally accurate and well detailed.
MTH calls their toy oriented products "Rail King" and of those product numbers begin with a 30-. Some of the Rail King freight cars are 1:48 scale. They are generally based on pre WW II cars that were of smaller size and do not dwarf more fanciful cars. Some Rail King steam and diesel locomotives are 1:48 scale and are advertized as Rail King Scale. MTH's highly detailed O scale models are in their "Premier" line and have 20- product numbers.
Atlas sells O scale models under their "Master Line" and "Traiman" brands. Trainman models are 1:48 scale but less detailed and lower priced. Atlas sells their toy train products under the "Industrial Rail" banner.
