What freight cars are appropriate for 1915-1925 era?

Any of the USRA designs would be appropriate.  After that it would be car manufacturers and fleet owners like Pullman and Railway Express (REA) and Pacific Fruit Express (PFE). And railroad specific designs like PRR GLa hoppers.

Jan

Outside braced wooden boxcars, small plain 8000 gallon tank cars, 36' refrigerator cars, 2 bay coal hoppers, wooden style cabooses.

Double sheathed and outside braced boxcars. Billboard wood reefers with private owner advertising.

Railroad historical societies have detailed information about their car fleets in different periods, but you would need to dig in to translate roster information into the appearance of specific classes. For example, I belong to the B&O RR Historical Society. I have the B&O's Summary of Equipment for 1920 listing every type of car they owned. They had 35,715 boxcars in dozens of different classes. Most of these were wood cars with either steel center sills or steel underframes, but there were still quite a few all wood cars. The 25,009 gondola were a mix, but all steel cars were present in the thousands. The 30,748 hoppers were almost all steel.

It depends how interested you are in learning about freight car history.

Would the 40' wood-sided reefers that a lot of manufacturers sell be appropriate, or would they be a later iteration of the 36' ones mentioned above?

I find research an enjoyable side hobby as I’m focused on a certain time frame to.  I generally rely on Atlas Masterline cars. I have some faith as far as it being accurate. If I see a car in a desirable scheme. I look for the BLT. Date on the cars side if I’m not sure.
Where you are focused on a certain time frame. The USRA cars would fit into your scheme. One thing to remember though. If you are also into doing weathering. You have to dial back on it a bit seeing in your 1925 time frame they would still be somewhat new.

@Dave_C posted:

One thing to remember though. If you are also into doing weathering. You have to dial back on it a bit seeing in your 1925 time frame they would still be somewhat new.

Although I would imagine it doesn't take cars long to get filthy on a steam-era railroad.

There a few pre-depression era railroad and early 20th century groups on Facebook.

They provide many photo examples of early railroad equipment.

These are pre-1914 cars I have modified and painted, from MTH 34’ 19th Century cars..

36’ cars would also be appropriate…These are factory painted, but modified by me…

Tom

If your goal is to have a layout that looks appropriate for 1915-1925, why would you be populating it with rolling stock that was just starting to come into use during that time period?  My information indicates there were 25,000 USRA single sheathed 40' boxcars, 20,000 drop bottom gondolas, 25,000 double sheathed 40' boxcars, 25.000 55 ton twin hoppers and 5,000 70 ton gondolas built for the USRA starting in 1918.  I'm betting that is a drop in the bucket of the total rolling stock operating on U.S. railroads in 1920.  For example according to information in the designbuildop.hansmanns.org website a majority of the B&Os boxcar and gondola fleet in 1926 was built before WW1, as was a large portion of its hopper fleet.  If the goal was to replicate the feel of a ca. 1920 railroad the starting point should be rolling stock that was coming online in 1910.

Another step is to be mindful of your road names.   During the 1910s to early 1940s you have subsidiaries that were rolled into their owners, companies with name changes in reorganizations, takeovers and mergers.  For example The Reading Company was created in 1924 to take over the Philadelphia & Reading Railway. 

Steel cars were starting to be introduced in the 1920's.  The PRR X29 was first built in 1924 and over 30,000 cars were built by the PRR alone, not including the other roads that used this design or made close copies of it.  Atlas O offered the X29 in many schemes.  Here is my PSC brass PRR version in the as-built appearance and detailing.  Plate ends and three panel doors with the brake weel on the top.

The PRR N5b was the first steel caboose (Cabin) in PRR talk and was also introduced in the mid 1920's to enhance crew safety.  This is Brassworks model with the postwar trainphone antenna system; however Lionel has made a very nice version of this in 3-rail that is era appropriate.  What many do not realize is that these predate the wood N6 designs of the PRR by a few years.

Collison posts on the end are also a later modification.

I only have partial photos of my Atlas CNJ X29, but as you can see it has the original doors, but not the original brake wheel location.  This represents a slightly later build of the car.  Perhaps late 1920's, but more likely early 1930's.  Either way, a very believable steel car for the 1920's era.

The only reason I post these, is that it helps to do some research as steel cars would be on many roads by 1925 even if the wood sheathed cars were much more common.  One of my favorite wood sheathed cars is the Atlas shingle sheathed USRA design.  Lots of these were built.  Weaver, Lionel, & MTH also have done lots of these cars in many roads and in 3 rail.

San Juan also made a really nice 1920's era plastic tank car that is era appropriate, but they aren't that common, and I believe they are all 2-rail cars.  Tank cars were very small during that period of railroading.  I don't have a photo of mine.

The PRR GLa hopper was also mentioned.  Here is a great link to GLa class hoppers.

What road name do you have in mind?  Sounds like a fun period.  While I am like many and enjoy the transition era a great deal, I have all sorts of reasons why some antiquated equipment that dates to the era you are interested in could still be around in the mid 1950's.  After all, UdeY, in Mexico was running standard gauge 4-4-0's up through 1963.