My guess
Oregon_Trunk;
I'll state up front that I don't really know the answers you're looking for. Therefore my guesses, (and that's all they are) will be generalities, not specifics, and some may be wrong. I have found however, through my personal experience on the forum, that posting anything that is wrong can often be an effective way of getting information. Six people write in to tell you what you got wrong, and their idea of what's right.
Your time period predates the US entry into world war one (in 1917) a little, so "composite cars" (minimal steel frames supporting mostly wooden bodies) probably wouldn't be appropriate. Composite cars were also called "war emergency cars" and the purpose of their design was to "save steel for the war effort."
All-steel "heavyweight" passenger cars would have been a fairly new innovation back then. Some class 1 railroads would have some, and they probably would have been used on the crack passenger trains of that time. Secondary trains, and likely all trains on a secondary line like your Oregon Trunk, would be more likely to be still using wood-sided passenger cars. There were perhaps exceptions, possibly including the personal car of a "Railroad Barron", for example.
Generally freight cars would be 40' or less, in length, and of lower capacity than those of even 20 years later. Vertical mounted brake wheels would likely have been common. Wooden roof walks would have been a normal fixture on all boxcars, cabooses, and stock cars.
Automatic air brakes hadn't been universally adopted by every line yet, and even those companies that were using air brakes would have left the roof walks and vertical brake wheels in place for setting the hand brakes on cars spotted on sidings.
Specialized grain cars, like covered hoppers, wouldn't have been around back then. The common method of shipping grain was in boxcars, with the grain shoveled in by hand. Perhaps using low partitions called "grain doors" to hold the load in when the hand shoveled load got close to the main doorway.
The most common freight car was the forty foot boxcar. Most freight would have been shipped in boxcars. Most hopper cars, and gondolas, would be shorter than the boxcars. Thirty six feet was a common size.
Log cars could have been anything as far as those odd little gadgets used to haul large, raw, logs from the woods to the nearest sawmill. They were often built by the lumber companies that owned them, and modified on scene. Disconnect log cars, and skeleton log cars, were two common types.
Logs would be a rare item on a conventional freight train, traveling a longer distance. If they needed to be shipped this way they would be loaded on conventional flat cars equipped with side stakes and the logs would be chained down. If you've ever seen the train wreck scene in the old film, "How the West Was Won", you know why!
Finished lumber was commonly hand loaded and shipped inside boxcars. If the Oregon Trunk freight agent knew his job, he wouldn't be likely to ship lumber exposed to the frequent rains of the Pacific Northwest. And, of course those neat plastic-wrapped bundles we see riding flat cars today, wouldn't have existed then.
"Cattle cars," or stock cars, would be 40' or less in length, and have the familiar open slat wood body, riding on a steel under-frame. Steel slated bodies were not built until much later.
Motive power, all steam. No diesels yet. A few electrics roaming here and there, but those were rare exceptions, and only on a few lines.
Finally, although, as he said, it might not be welcome news; I have to agree with the part of CTValley's response saying that only exhaustive, personal, research of the prototype railroad, not the scarce available models, will get you accurate information. My preceding guesses are based largely on my reading about the Milwaukee Road's activities in the Pacific Northwest in a fairly close time period.
You have set yourself a very difficult task. If you were modeling the Santa Fe, or Union Pacific, there would be a lot more material generally available. I found that even the Milwaukee Road, which was a class 1 transcontinental, has had much less written about it than many other railroads. Modeling a subsidiary of the SP&S, which was itself a subsidiary of the Great Northern, has to be a formidable research challenge. There is a plus side though. I found the "detective work" of researching a prototype to be quite interesting in itself. I hope you will too.
Good luck, have fun;
Traction Fan:smilie_daumenpos:
