NG stock cars in the Texas

While visiting that great Mecca of narrow gauge steam in Texas, Six Flags over Texas; their canned narrative mentioned that their rider cars began as stock cars in Colorado. Does anyone know the origin of these cars?
I have been going to Six Flags since the mid 60's to visit cousins who lived in Arlington. One of my fondest memories was of a late night cab visit courtesy of the locomotive engineer. He let four boys up in the cab, and then took great care in showing us around. I have been hooked on steam ever since.
There was a pretty young female engineer on Sunday (nothing wrong with that), but the way we were herded on and off the train, there was no time for questions or pictures or anything. How are we supposed to get the next generation interested?
I shouldn't gripe; at least they are running real narrow gauge steam engines. Both engines ran (#2 on Saturday and #1 on Sunday); other amusement parks in Texas have gone to the "steam" outline engines that have "dismals" installed under their tenders. Enough said.