Lionel postwar included a lot of freight cars (mostly in the 027 line) that were exact 1:60 models as far as the bodies were concerned, altho they rode on 1:48 trucks. The Baby Ruth boxcar of the early Scout (1948 in my case) sets were even smaller in the body-- I assume 1:64 on this one.
A car that everyone would have assumed, being included in premium O sets, to be a "scale" model of a 40' boxcar at 10"+ in length (vs the earlier 8"+ boxcar models of 40-footers at 1:60 on 1:48 trucks), was the well-known boxcar 6464 and all its variations. These cars were introduced by Lionel in 1954 IIRC, and interestingly the railroads introduced the 50' boxcar about this year.
The reason why the 6464 lacked the height and width of a 40' scale (at 1:48) car is that they were actually models of a 50' boxcar at 1:60 as far as the body went. Some say the width was based on 1:55, but actually these cars were 10'-8" wide in the original.
All of these questions can easily be checked with the two 12" triangular rulers cheaply available in stationery/office supply stores. There are two basic styles, called respectively an engineer's scale and an architect's scale.
The engineer's scale originally included 6 ratios on its six faces: 1:10, 1:20, 1:30, 1:40, 1:50, and 1:60. (Variants are now found with 1:25 in place of 1:60, which is also suitable for this purpose.) The 1:40 scale is used as a 1":4' scale (where 1" will measure off 4 feet on the 1:48 model in feet and tenths of a foot). The 1:50 scale is used as a 1":5' scale (where 1" will measure off 5 feet on the 1:60 model in feet and tenths of a foot).
The architect's scale has a regular 12-inch and sixteenths ruler on one of its six faces; the remaining five faces have ten scales on them, in pairs running (the numbered feet) in opposite directions. The smallest of these is 3/32"= 1 foot; the largest 3"= 1 foot; those on each face are usually paired in a 2:1 ratio, for example the pair to 3/32" is 3/16"=1 foot, which you will want to use for S gage at 1:64. There is also a 1/4"= 1 foot scale which you can also use for O scale at 1:48. Here you have the advantage of having the inches exactly on the architect's scale. Each end of a face has an additional foot before the zero mark, which foot is subdivided into one-inch markings. This enables both feet and inches to be read off the model from one position of the scale.
The plastic versions are relatively quite inexpensive. The professional will want the engine-divided faces on boxwood, which are most accurate; these can be difficult to find and somewhat expensive. Unless you are making the cars, or are a real rivet counter, plastic should do fine.
--Frank