Steam

[doctorwayne]

Harry, pretty-well any wood car (except those with truss rods) could have lasted into the mid-'50s. Some were built as late as the WWII era, in an effort to conserve steel. While I'm not familiar with what's available in N scale, the 1937 AAR-designed steel boxcar was quite common, as were examples of the proposed ARA 1932 design. Pretty-well any car with a lower roof line than the preceding two examples would also be good for your chosen era - this includes the Pennsy X-29 style cars, USRA steel boxcars, and early ARA steel boxcars. Also, the PS-1 boxcar was introduced in 1947, I believe, along with other "modern" cars. Most boxcars were 40' or 50' in length, although there were still some 36'-ers around, too. Reefers usually 40', steel or wood construction, although lots of older 36' and 38' cars survived because of the spacing of the doors at shippers' docks.
Tank cars, for the most part, still had frames, and were generally 40' in length or shorter. Flatcars were of riveted, welded, or cast construction, and generally 40' to 54'6" in length. Gondolas could be composite cars, with metal underframes and bracing, and wooden sides and floors, or all-steel types, of either welded or riveted construction. Again, lengths 40' - 50'-or-so, with some up to 65' by the mid-'40s.
Squidbait's advice on BLT dates is good, and he's right (as usual): time to start reading up on your freight car history.
Since I am unfamiliar with what's available in N scale, below are some examples, in HO, of cars that would be suitable for a mid-'40s to mid-'50s era layout. Many of my paint schemes, though, are of 1930s' or earlier vintage - after WWII, lots of boxcars got fancy heralds and slogans and many cars received paint schemes other than the ubiquitous boxcar red. That's one reason that it's the most popular era to model.

This hopper is a pre-WWI car, but served into the '50s:
[album]1009[/album]

Here is a USRA-designed hopper, first built in 1918, I believe, with thousands built in the following years for railroads all over North America:
[album]830[/album]

This is a rebuilt USRA hopper - as the sidesheets rusted-out due to the corrosive nature of coal, many roads rebuilt their fleets with these stamped panels, which were available from various car manufacturers, introduced in the early '30s. They also increased the car's cubic capacity:


The AAR offset-side hopper was introduced in the early '30s, with 2-, 3- and 4-bay versions available:


Covered hoppers, usually shorter than 40', first became common in the mid-'30s, and were available, with some minor variations, from several builders:


This 41' gondola is a 1941 AAR design, common everywhere:


Similar cars in longer lengths were built by all of the major suppliers, in riveted:


or welded construction:


There were also "composite" gondolas of USRA design, from 1918 and later (some roads used the same outside bracing, but with steel sides instead of wood):


During WWII, the so-called "War Emergency" cars were built, reverting to the WWI-era USRA designs, in an effort to conserve steel:


Flatcars, usually 40' or 50', were steel with wooden decks. Construction could be riveted:


or welded (of which I have no examples) , or cast, like this one:


Wood-sheathed reefers were common well into the '50s:


And steel ones, introduced in the '30s, well beyond that:


As noted, 36' and 38' wooden reefers were also common into the '50s:


For boxcars, common lengths were 36', 40' and 50', although there were also longer cars around. Inside heights varied from about 7'9'' to 10'6". This modified Athearn boxcar is similar to the 1932 ARA design - this car has been offered in probably every scale that's been around since the prototype was built, including N:
[album]561[/album]

These two cars (note the disparity in heights) are both products of the 1920s, but could have lasted into the '50s:


There were single sheathed cars in various heights:
8'6"

10'

10'6"

And various lengths:
36'

40'

and 50'

And ditto for the double sheathed cars, although I have only 40'-ers:


Both wood and steel cars, usually in 40' and 50' lengths, could come with single doors:


Double doors:


or as door-and-a-half cars:




Also common were single sheathed wooden cars, such as this:


which were re-built with exterior steel sheathing, such as this (you can still see the bottoms of the exterior bracing showing at the side sills):


That's probably more than enough examples to get you started on your search.

Wayne