1970s

From what I found by goggling is they came out around 1970 but I did find that the PRR X60d was a 64' HiCube that came out sometime between '63 and '68. I think the build date will be determined by the road name. 

My 2 cents 

Which Genesis Hi-Cube Boxcar?

"High-cube" boxcars have been built since the 1960s for auto parts service, but more recently pretty much all new boxcars are high-cube designs, either 50-60' (inside) length.

Athearn has done some 1970s era auto-parts cars, but also some modern 1990s general service high cubes. The above response may be accurate, but your car likely does not represent one of the earlier designs, so it may not be entirely relevant to your particular model.