My layout spans 1900 - 1939, as far as buildings and cars and trucks go. Of course, buildings can be any era, except modern era architecture does not fit well when running steam, in my view
I stretch it a little with my little SW-1 Switcher, which does the very slow speeds in a commendable manner. It is my preferred locomotive, and they were first built in 1939. I also try to do some prototypical Northern Pacific trains, using road names common to this part of Minnesota.
I tend to be a little anal about getting "year built" on box cars right!
That being said, I have several transition era Diesels also cabooses and freight cars in storage. I have been tempted to bring them out but when push comes to shove, I have enough of the earlier stuff to keep me entertained.
I keep what I believe the minimum number of freight cars, and keep less than 1/2 of them on the layout at a time. I have sold 3x more cars than I now own, just to keep things simple and make the best use of my storage space.
Like
Mara, most of the trains that I have now have some sort of emotional attachment that I don't want to get rid of them. My Soo Line truss rod box car is a favorite, in part because of the old photo I have with some relatives
Here is the box car I have, not exact but same logo:
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Here is the photo I like, in front of the Hinckley MN depot, circa 1890?
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I am kind of rambling but to answer, I mostly keep to one era.
Dave LASM