I can be as good at research at anyone!
The remarks about King Solomon above are exactly on target.
You can look at the Built or Repaint/Shop dates on every single freight car you pick up. You can determine the end of the steam era on your railroad, real or imagined and set a month and year in time as your final cutoff date for steam operations.
However, what happens, or at least what happened for me personally, is only heartache.
Excepting the plethora of Pacific Fruit Express Reefers, the number of TRUE 100% steam era freight cars available on the market is NOT whatever you think it is. In MANY cases, the 40' boxcars readily available from Kadee or elsewhere represent repainted cars that do NOT belong behind steam, even if you are modeling Colorado & Southern during fall of 1962 (the last gasp of mainline, Class 1 railroad, steam operation in America, with only one 2-8-0 still running.)
Those beautiful Rutland PS-1 green and yellow boxcars date from 1957, after most railroads had abandoned steam. The Blue and Black B&M boxcars, from 1956. The hot yellow Kadee ACY boxcar is a 1964 repaint of an earlier freight car red boxcar, and was limited to very few yellow repaints (perhaps only ACY 745). The red GB&W Kadee 40' boxcar is a 1968 repaint, and on and on it goes. The vast majority of Kadee 50' boxcars do not belong behind steam at all, are 1957 and later boxcars.
Excepting the refrigerator cars, most steam era freight cars were brown, black, or occasionally silver (tank cars).
Most Intermountain ATSF stock cars are post-steam era rebuilds and repaints. Only the early ones belong behind steam. Many Intermountain ATSF reefers represent post-steam era repaints.
People like myself, born during 1968, tend to prefer more modern freight cars including the colorful boxcars of the 1960's and 1970's. It can be difficult to rationalize putting any of them behind a steam engine, or even early diesels (a lot of Alco road switchers were already going to scrap by the mid-1960's).
It's my railroad, and I run what I want to run on it. Unfortunately, that means most of my freight cars are averaging 1 decade or more too new as compared to the factual last dates of operation of my steam power in real life.
P.S. I cannot tell you the number of times I have been in a store and picked up an interesting new freight car, contemplated buying it, and then put it down realizing that it was just way way too new for "transition era" modeling, at any stretch.
One of the reasons I sold so much plastic recently was the realization that virtually my entire freight car roster was so implausibly modern as compared to my steam power (Rock Island 2-8-2's scrapped during February, 1953, and T&P steam, scrapped by 1952) that my trains were somewhat of a "joke".