Not all of Colorado was full size couplers. The Colorado & Southern, Colorado & North Western, used 3/4 size couplers. The Florence & Cripple Creek and Uintah Ry did as well I think. My guess would be

After reading this very informative thread, I ran across a picture of three of these tank cars helping get a satellite into orbit tonight. Speaks volumes about how well they were built. (And for steam