Most heavyweights in the latev19th and 20th century were 80'.Were most heavy weight cars just 60 feet long instesd of today's 80+feet long? Today's cars just have 2 aels per truck as the old ones had 3 axels per car But then the REX, some RPOs and storage mail were even shorter.
Mayb the number was more what one loco could pull on flatter track and the helpers on steepe locations?
60' cars were more 19th century.