Lot of 8 Proto 2000 freight car kits for less than $8 each including shipping, which is a deal.
These are quite nice, highly detailed kits that usually run at least $20. They have separate (and very fragile) ladders, grabs, steps, and other detail parts, aimed at adult modelers. The similar cars have different road numbers too, so that won't have to be dealt with later on.
Alas, a couple have been partially started, but the work is mostly OK and what's not can be camouflaged with weathering.
The five drop-end mill gondolas are appropriate for a steel mill setting.
The double-door boxcars can load either complete automobiles or, more commonly, stamped metal parts for automobile construction, so they're appropriate for a sheetmetal parts-factory setting, which could be close to a steel mill.
No real need for the stock car, as feedlots, meat packing plants, or cattle loading pens don't figure into my plans, but you never know.