Look at the build dates on

Look at the build dates on the side of the model as well as the reweigh dates

Dates

Eric Hansmann has a lot of good information at http://designbuildop.hansmanns.org/2016/08/06/decals-and-lettering/

The pictures blow up to full screen if you click on them.

HO models

Pretty much if its not a USRA design of has an IH of more then 10 ft, its probably too new.  Look at the Westerfield Models website, he specializes in your era and gives build dates for the models.  Even if you don't want to build those kits, they will show you what the cars look like.  If the model has Andrews trucks its probably  in your era.  The biggest and brand newest hopper you would have are the Athearn offset quads.  The offset and rib sidied twin hoppers would be appropriate.  Ironically the composite wood sided hoppers (like the Athearn one) are too  new for your era (they are 1940's cars).  

Except for hoppers, all steel cars would be brand new.  Gons would be mostly in the 40 and 46 ft length, a few 52'6" and very few 60-65 ft cars.  Steel cars would be riveted and not welded.

Eric Hansmann's Website

Eric Hansmann also has a guide to 1920s plastic freightcars:

http://designbuildop.hansmanns.org/a-guide-to-1920s-era-ho-scale-plastic-freight-cars/

I'll keep an eye out for

I'll keep an eye out for those. Thanks. It just seems that not all companies include that info. 

Excellent. Thanks for the

Excellent. Thanks for the tip.

Great advice. Thank you.

Great advice. Thank you.

Book

The Model Railroader's Guide to Freight Cars by Jeff Wilson (out of print) could help.  It covers WWI to the present.

Paul

Sweet!

Sweet!

The Westerfield site and

The Westerfield site and Hansmann's site are excellent resources but also check the dates printed on the sides of the cars as suggested by Gramam Line.   Even if the car is correct for your era, it may, but not necessarily, have a paint scheme that is too new. If the date is too new, further  research is necessary.  A car with an out of period date would be acceptable if the paint scheme is otherwise correct and the date doesn't bother you. Alternatively you could change the date. 

Another problem with the dates is the model manufactures may use fictional dates. Fortunately this is less prevalent than it used to be.

Definitely look at prototype photos in books and on-line.  While probably  most are not dated, it may be possible to read the dates on the car , although to do so may require magnification. 

Accurail

On Accurail's websites, they include the build dates of each car.  Some will have a couple of build dates, but if you telephone to order, instead of ordering online, the folks there are wonderful, and will get you cars  with the right date.  Very helpful people.  (Since you are calling a working factory IN THE USA! there might not be someone available to answer the phone right away.  Good to see Made in USA!)

Go traditional...

The traditional dinner and a movie still work, just be sure to pick a restaurant and theater close to the tracks. 

Randy

@Randy

Was just waiting for someone to do that...

Stuff we find...

Thanks for the DesignBuildOp blog mentions in this thread! I enjoy sharing rail info from the Twenties so modelers can understand the railroads and industries of that decade.

As far as tips for dating rolling stock, start keeping notes. That is how the guide to 1920s era plastic freight cars started. As more modelers inquired about the info, it became important to post it as a resource. I would invest in a mid-1920s Official Railway Equipment Register (ORER) so you gain an understanding of the in-service freight cars on several railroads. I summarized the 1926 Wheeling & Lake Erie fleet when that was my focus. I'm halfway done with a B&O version and hope to post that soon. The larger the railroad, the more detailed the summary.  

Your interest in Harlan & Hollingsworth is interesting. By the 1920s, the company seems to have been a subsidiary of Bethlehem Steel and mainly a shipbuilder. Check out this page of the car builder history for more. There would be lots of interesting inbound loads to a shipyard but few outbound shipments. 

Become a sponge for information and take notes. I find the research into the freight cars of the 1900-1930 years to be fascinating. Feel free to contact me directly through the ID here or via a comment on my blog. The 1920s are a fascinating era for rail and industry that have had little coverage in the hobby press. 

Eric

equipment rosters

Any particular railroad's equipment roster can be useful, but there will be other cars on the line, of course.  Old photos can help there, to get the right mix.  Location's important, too.  At least today, 20 to a hundred miles can drastically change what you see.

Dover, NH:  Plenty of classic manifest freights with a good mix of cars--no intermodal, coil cars, or lumber cars that I saw.

Rochester, NH:  When I'd see a train, it was usually a gravel train.

Manchester, NH: Coal trains for the power plant, with the occasional tank car.

White River Junction, VT:  Lost of wood cars, including raw wood as well as lumber, plus assorted others. 

None of the above, in my experience, ever had an intermodal, coil car, or autorack

Springfield, MA:  A lot of intermodal, some coil cars, as well as assorted cars of all sorts.

H&H

Harlan &Hollingworth was primarily a passenger car builder.   I haven't really seen any freight cars they made.

Eric's Blog: Highly Recommended

I enjoy reading Eric's blog, and anyone interested in modeling the 1920s or 1930s should check it out. 

I also look at http://steamerafreightcars.com/,but it doesn't seem to be active anymore.  Still tons of interesting data.

I'm drumming up an old topic,