The word "average" is not a good one, because some 50-footers and some 60-footers have been among the most recent ones built, and the "average" would probably be something like 58.4 feet, a totally meaningless figure.
Perhaps "typical" is better.
Just about all recent box cars have been of the high-roof variety, with an inside height around 12 feet or so. Most of them have an inside length of just over 60 feet, though quite a few have been built with an inside length of 50'6". That, by the way, was the inside length of virtually everything built during the box car boom of the mid-1970s through 1980. There were a few hundred built to an oddball length of 52'6" or so (as precise as I can be without a trip to the dungeon), as well as a few 60-footers.
The hi-cube auto parts box cars haven't been built in any quantity since the mid-1970s. One group was built recently, and another single car--narrower and taller--was built a few years back and not duplicated. These cars are on their way out, due to their age--and you'll probably not see them repeated. But all of the hi-cube cars had an interior length of 86'6", and most of them had a capacity of 10,000 (exactly!) cubic feet.
Carl
Railroader Emeritus (practiced railroading for 46 years--and in 2010 I finally got it right!)
CAACSCOCOM--I don't want to behave improperly, so I just won't behave at all. (SM)
