Hi,
  I don't know if it is true - or simply a persistent and often repeated rumor - but I've
heard many times from many different sources (model RRs) that one of the 
reasons for the GN doing double-sheathed cars was to be 'friendly' to its many
online customers that made or sold that product.  The only reason why I 'waffle'
on the veracity of that is because I've never seen anything that even remotely
resembles a "substantiating fact" such as an agreement between GN and a
customer.  Nor, for that matter, not even records of the GN receiving plywood
from its customers.  Did they - quite probably ... just saying I haven't seen the
hard evidence.
                                                                   - Jim in the PNW