Boxcar length's of 40 ft. There are a few 50 ft cars out there.
Boxcars with an IH of 10 ft or less.
Boxcars with dreadnaught or Youngstown ends.
Boxcars with panel ends.
Gons in the 40-46 ft range.
Cars with rivets (not welded).
Boxcars and gons in the 40-46 ft range with composite or wood sides. (Composite hoppers are WW2).
Covered hoppers extremely rare.
Tank cars riveted.
Hoppers would be twins or quads, ribbed or offset sides. 50-55 ton twins. 70 ton quads.
Dave H. Painted side goes up. My website : wnbranch.com
For "house cars" (boxcars, reefers, stockcars) they'll generally be lower than later cars. The 40' long, 10' 6" boxcar didn't become common for new construction until about 1936, and wasn't a "typical" boxcar until the 1950's. Most cars c.1930 were 8' 6" high. 40' long cars had been the standard (except for some shorter meat reefers) in new construction since about 1915, but some 36' cars would still be around.
Archbar trucks and cars with trussrods (unless they had a steel centersill) were banned from interchange service by 1938 IIRC. Refrigerator cars with "billboard" lettering were banned (in effect, it's a long story) about the same time.
Most cars would have wood sides, ends and roofs. Car sides and ends could be double sheathed or single sheathed, the latter would have outside-bracing of steel. Some cars were a composite - wood sides, steel ends and roofs.
The problem with going by when a car was built is...well, it only tells you when a car was built. You need to become familiar with the lettering styles of the railroads also. A Pennsylvania RR X-29 steel boxcar built in 1928 would be perfect for an early 1930's layout - unless the model is lettered with Pennsy's 1950's or 60's lettering. That's true for passenger cars, many Walthers heavyweight steel passenger cars are correct models of cars built in the 1910s-20s, but are wearing post-WW2 "streamliner" paint schemes.
one rule of thumb is that earlier cars were shorter. that being said, get on the web and look at all the photos you can find. shorpy is one of many good sources. if the photos are dated, and many are, you will see just what you need to look for.
Charlie
I'm just starting over in HO and am modeling the period up to about 1930. When looking at ads for rolling stock, are there some telltale signs that will indicate if the model is not too "modern"? Often, I'm happy to say, the ad copy will mention the date when a car was first manufactured, but to save me time, are there signs that I can look for, such as composite sides or vertical brakewheel staff or--well, you tell me. Thanks.