" I am having difficulty

" I am having difficulty finding a definitive list of car types typically seen on the UP line during that period." 

   Rail magazines and videos from that location in that era are a good place to start.    what exactly are you planning to model, a branch line or a mainline or what? The cars seen will vary depending on the traffic to local industries and thru town on a mainline. .DaveB

Build dates are also useful,

Build dates are also useful, you obviously won't see anything newer, and generally won't see anything older than 20-30 years, with exceptions based on local industries. For example I model NAR in the late 70's, which means tons of 30-40 year old 40' boxcars and often in schemes 20+ years gone as these were used in grain service on light rail lines like the NAR where there still was plenty of 60lb rail even in the 70's. Expect to see more home road than foreign with exceptions (NAR modeller here, 920 miles of track, 100 boxcars, no other revenue cars in interchange service.). Exceptions for UP would be local industries that either have private mark cars or receive significant shipments from off system. 

Pictures and a good idea of your local industries are the key. UP in your era will be relatively easy to get pictures of, you should have little difficulty getting an idea of what types of cars and what reporting marks you should be seeing.

Again, for a NAR modeller, shots of the trains in my era show a lot of 40' boxcars and cylindrical grain hoppers, a large subset of double-door 50' boxcars (lumber and pulp service), large strings of coal hoppers and ore cars in certain areas based on nearby industries (coal mines just north of Edmonton and ore trains from the GSLR) and plenty of tank cars on the GSLR run and on the Waterways run. Plus mixed trains, especially on the Waterways run.

But that's the NAR in general, for the sub I model there's no passenger service anymore, despite the NAR having mixed trains right through the CN buyout in 1981, and no coal or ore service (that was on different subs). So for me it's grain and lumber & paper as the primary outbound shipments and equipment & fuel inbound, with a small amount of through traffic, either from other NAR subs to the ARR and BCR interchanges.

The same is the case for power. Again, getting back to the NAR it had GP9's, GMD-1's and SD38-2's. For the sub I model the SD38-2's never came that way as they were used on the heavy grades of a different sub, GP9's were standard and GMD-1's were occasionally seen (but mostly were on a different sub as they were for very light rail & passenger service). But I also need some CN and CP GP9's as they were regularly borrowed for extra power, mostly CN at that point. 

Info on rolling stock

Build dates

Official Railway Equipment Register

Pick up a copy of the ORER for your era, and you will have an exact listing of car types and numbers for all cars in interchange service.  You can find them on Ebay.

Infinite Choices

Using Google

Might take a little time but I have used Google many times to search out info like this. Knowing how to ask the question, many times, is the biggest challenge. Eventually you find a good combination of words that works. Most of the time, I come up with a lot of links. Have to be careful as many times I find a discussion and I get opinions. Have to look for facts.

Rich

Build era drives rolling stock

Another Source

If you Google "Railroad Freight Car Photos", you will come up with a wealth of on-line photo archives.  One I have been using lately is railcarphotos.com.  This site gives you the ability to search by location, date range, car types - nearly anything.  By narrowing your search parameters, you can get an idea of what cars were in your region (well, OK, in the state anyway) in your era.

Haven't tried this with some of the other archive sites, but you can probably do the same type of thing.

"I plan on a mainline with

"I plan on a mainline with maybe an oil refinery and a cement facility as main industries. I think that the UP from San Antonio west to Arizona will offer me a lot of industries to pick from. I would like to perhaps run an Amtrak passenger that should not look out of place for the late 90s"

       I'm not sure about the UP timeframe taking over other railroads in Texas but they didn't take over the SP  until 1996.  As for cement plants the cars would be 2 bay covered hoppers for the cement and maybe coal coming into the plant in open hoppers. 100 ton cars would be suitable for the 90's era, probably pretty well decorated with graffiti by then.  A refinery would use lots of tank cars most likely black ones:> ) . Another common car in many areas would be lumber flat cars or boxcars delivering to local lumber yards or distributors. If there are agricultural areas nearby they might also use some reefers and some chemical  covered hoppers or grain hoppers if there are any grain or beans grown along the line. If it's dairy or poultry country covered hoppers of feed would be needed. Most of these cars plus more( intermodal cars, steel cars, auto cars, etc)would be seen on a typical mainline route. It's possible to limit the type of cars needed by picking a branch line if the space is small. In the 1996 era the branch line here only saw coal and cement cars serving the cement plant plus lumber cars to a lumber yard, and very rarely an odd load to a team track or a maintenance of way car for railroad use. .DaveB

which '90's

Until you mentioned cement, I had visions of ten-wheelers and 4-4-0's running around...I might have been able to find some answers then.  But the 1980's and 90's....I know less.

Railpictures.net is a great resource, though.

Cars

Lots of questions you have to answer.

1.  Does my railroad really go there in the era I am modeling?

The UP didn't run between San Antonio and Arizona until after the merger with the SP, so if you are modeling an era pre-merger you will have a really hard time finding examples.

2.  What cars were operating in my era?

An Official Railway Equipment Register (ORER) will list all the cars by number and mechanical characteristics.

3.  What models match those cars?

Much harder question if you care that the cars are accurate.  You will have to track down pictures of the cars in which you are interested and compare them to the models or track down pictures of the models to see if they are the accurate types of cars .  Model companies sometimes put the wrong car number on a model (gasp!).  There are also issues of how the cars are painted (if you care) a model might be the right type and number, but the paint scheme may to early or late for your era.  Once again search for photos if you want that level of accuracy.

4.  What cars operated on my territory?

A hard question also.   Different routes have different mixes of cars.  For example in Ft Worth, the trains on the UP's E-W line will have a much different mix of cars and trains than those on the UP's N-S line.  Look at pictures of trains on your territory to get an idea of the types of trains.  Just realize that most pictures are of the engines and head 20 or so cars of a 100 car train.

5.  What industries are on my territory?

Also requires a little work.  Best sources are track charts or Zone-Track-Spot charts (ZTS on the UP,  CLIC on some other roads).  I'm sure that if you pick any 1000 mile stretch of railroad you can find enough industries to put on a layout.  Just remember San Antonio to Arizona is mostly across some pretty lonely country and there are industries but outside of San Antonio, El Paso or Tucson, they will be spread out many miles apart.  There are lots of 40-50 mile stretches with no industry at all, just rails across the desert.

The good news is that the UP is a huge railroad so the chances are just about any type of car could be there.  That route is big on intermodal, auto and manifest (general freight) low on bulk (coal and grain, except on routes that connect with Mexico).

rolling stock

In choosing your rolling stock, a difference of a few hundred miles can make a huge difference.  My personal experience is New England, but here's an example.

In and around White River Junction, Vermont, you see manifest freights, with all sorts of cars, including a lot of wood and paper related cars.  Drive a couple of hours to Dover, NH, and the manifest freights are running still, but with more tank cars and no wood cars.  (Incidentally, even today, you can still see empty boxcars running with open doors.)

Rochester, not far away, there's gravel trains in abundance.

Head to Boston, and you can see almost anything.  Springfield, Massachusetts, when I've passed through, I've seen autoracks, coil cars (which I've never seen north of there...) and almost anything else.

So, traffic varies with even a short distance of travel.

Great responses! I will try

oops...

I somehow posted this to the wrong place, and can't delete the post in its entirety--sorry!