Which ones?

The first batch of open air cars were the 9600 Pipe Gons. But these Pipe Gons were cut down from Standard Gauge Fowler Box Cars in the 1950's. I think these outside braced cars were originally constructed in the 1910's or so. So built in 1910's. Cutdown to Pipe Gons in the 1950's and converted to Open Air Gon in the late 1950's and reconstructed with roofs in the Mid-1960's.
After the need for more of these open air cars, more standard gauge BoxCars and Stockcars were scrounged up and converted over the years. I think there are two un-converted Standard Gauge Stock Cars in the Durango yard.
Those are the basics, if you need more information, like car numbers etc, just ask
Kevin Cook