If you can find them, the old Train Miniature steel boxcars are pretty decent starting points for USRA and ARA steel cars. They're now made by Walthers, but I don't know if they're currently available. I also like their model of the Pennsylvania RR X-29 boxcar, and was lucky enough to find 3 of them at the LHS yesterday (Walthers version, new in the box.) As mentioned, the Accurail double and single sheathed boxcars are good, and Red Caboose does a very nice X-29. Proto1000 does a nice version of the 36' Fowler Patent car and Proto2000 offers a 40' Mather stockcar and a Mather boxcar. Both Atlas and Roundhouse have 36' reefers and Tichy and Red Caboose have nice looking 40' wood reefers. I usually find most of my rolling stock on the "used" table at the LHS, or, as was the case with the X-29s, pick up a bargain, as my LHS buys up both estate lots of train stuff and the stock of hobby shops that have gone bankrupt. With a little (or a lot on some) work, you can have some decent rolling stock and a lot of modelling fun for a reasonable cost. Here a pictures of a few cheap cars that I've modified to appear more prototypical. Most of them have also been custom painted and lettered with decal or dry transfers. There are also a couple of not-so-cheap ones included too, as we all need a treat once in a while.
Roundhouse 36' reefer, made into a boxcar with the addition of a door. A few added details, this one has been downgraded to work train service.
Walthers 50' single sheathed automobile car, on the left (I picked up 9 of these, undecorated, for $5.00 each) and at right, a Red Caboose X-29 style car (5 of these at $8.00 apiece :thumb: )
Accurail 40' wood reefer
Accurail 40' 9 panel boxcar
Accurail 40' 9 panel boxcar (with a half-door modification from New England Rail Service added)
Athearn 50' wood express reefer
Red Caboose X-29 boxcar (the Pennsy owned over 30,000 of these, and they were seen all over North America)
Train Miniature (now Walthers) X-29 boxcar
Train Miniature X-28 boxcar (modified X-29)
LifeLike (Proto-no-thousand

) 36' reefer (a buck on the junk table at the LHS)
Train Miniature USRA double sheathed boxcar (this car was a pretty major rebuild)
Accurail USRA double sheathed boxcar (this car is stock save for a few added details)
Stewart U-channel hopper
Tichy rebuilt USRA single sheathed boxcar
USRA door-and-a-half boxcar (built from a Train Miniature plugdoor boxcar)
ARA 40' steel boxcar (Train Miniature)
Proto1000 36' Fowler Patent boxcar. CNR and CPR owned over 70,000 of these cars, which roamed North America (these cars retail for between $30.00 and $40.00, but I was lucky to pick up 6, bodies only, for two bucks apiece)
Red Caboose 40' reefer
Train Miniature 40' single sheathed boxcar (I changed the ends on this car)
Another Train Miniature 40' plugdoor boxcar (shave off the plugdoor details, cement a new door in place)
Tichy 40' reefer
Athearn 40' wood reefer with steel ends
Milk/fish car from an Athearn 40' reefer (replaced the kit's steel ends and built a new roof with no ice hatches - this is a rebuild of the car pictured above)
Athearn 40' steel reefer (this car was extensively rebuilt to follow an 1939 prototype, but the stock Athearn car is also appropriate for your chosen era)
Athearn 40' steel reefer, mostly stock
Concor (modified) 52' welded gondola ($2.00 off the junk table

)
Intermountain refrigerator car
Rebuilt Athearn wood reefer
Intermountain composite drop-bottom gondola
Walthers USRA 42' steel gondola
Athearn 50' steel gondola
Proto2000 52' steel gondola
Athearn chemical tankcar
Athearn standard tankcar - this one has been shortened - the 3-dome car is also appropriate
Intermountain tank car
Athearn 50' flatcar
Walthers 52'6" GSC flatcar
Athearn 34' offset side hopper
Athearn 34' ribbed side hopper
Athearn 34' ribbed side hopper with side panels from Pike Stuff
Accurail panel side hopper
Athearn offset side 4-bay hopper (this one has been shortened to 3 bays)
Stewart offset side 3-bay hoppers
Stewart 55 ton fishbelly hopper
I hope that the photos aren't "too much", but at a 1,000 words per picture, that would've been too much typing for even me!!

My fingers thank you.
Wayne