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Rolling stock doesn't "match the loco". It's all about era and what timeline you are modeling. If you are modeling say post WWII, you won't be running modern intermodal and container trains... they haven't been invented yet. If you are modeling the present era, you won't have a Big Boy pulling a mixed freight freight train.
The railroad used equipment for decades before replacement. So figure a 1960's loco could be pulling rolling stock from the early 50's to the late 70's, or more, as an example.
Any roadname rolling stock could very likely be hauled by any roadname motive power.
Do some research. Look at pics on some of the railroad photo sites. Read older posts on this forum. Pick an era you want to model, and go from there.
Use Google, or whatever your favorite search engine is. Do the research. Buy some books pertaining to the railroad you'd like to model. Check out that railroad's historical site. All the info is out there. But only you can decide what you want to model.
It would depend on the "train" you picked. Look at the Santa Fe historical web site, the prototype manufacturers web site, or even Wikipedia, and look up the production dates and/or delivery dates of the model of the loco you like. Then figure rolling stock could be +/- 10 years (at least) of the production date of the loco. Then do the same for the rolling stock. You'll be close enough for government work!
Do some research to find out what RR cars were operating around the time frame {era} you are trying to model.
Also do research on your chosen Loco's "brand name" {UP, BNSF, B&O etc} hauling contracts and businesses served during your era.That will give you an idea of what type of RR cars they would haul.
You can also do individual research on RR cars by "build date", or "Rebuild date" to find out when they were in service.
It can also be found that some RR equipment was allowed to be "in service" {and often "rebuilt"} for up to 40 years. so a steel box car from 1950's COULD still be in service in the 70's and 80's, though inprovements can make older cars moot. WHen changing from all wood box cars to steel clad wood box cars to all steel box cars would have rendered the all wooden ones useless quickly as the steel ones were "better', 40 years of service or not.
Also if you are considering passenger service, some trains ran specific car sets on some trains. Some roads even used the passenger cars of other roads on long distance runs (UP/CNW among others).
There are also old freight car guides available (by the year) that will tell you what cars were available on what railroads. They are usually available at swap meets.
You need to know the years your locomotive operated and how the railroad actually used it. Then you can look for rolling stock that fits those years. Freight cars have manufacture dates stenciled on the side so it ought to be easy but the manufacturers aren't very helpful as they rarely put the build year on the cars or boxes in big enough print for you to read it. So you have to know what kinds of rolling stock fit those years and then look for those when you go shopping.
Seems like it ought to be easier but it ain't.
I like late steam but I also run UP 844. Back in the '90s UP had 844 pull all sorts of 'modern' freight - not so much now though. So, I can get away with pulling many more modern cars behing 844 than I can behind my 3985 or any of my Bigboys.
If you want to know for sure what cars were around in at a certain time, get an "Official Railway Equipment Register" often called an ORER. It lists all the cars in interchange service at a certain date. It also gives general dimensions for the cars. I found the ones I have at swap meets for $5 to $20.
As to what cars are it what trains, that depends on the start and end points of the train. The one that goes past my house has lots of logs and lumber products, a few covered hoppers, fewer tank cars and even fewer box cars. It doesn't have any intermodal or coal cars. A transcontinental main line will have more variety and hopefully more trains