Greg
MTH makes a very nice modern 60' flat car. They have offered them in the 2015 Vol I catalog with track loads.
The Atlas Trainman 62' bulkhead/68' standard flat cars are good models too.
For more info see both parts of the O Scale Freight Car Guides to Flat Cars:
https://ogrforum.ogaugerr.com/t...piggy-back-flat-cars
https://ogrforum.ogaugerr.com/t...heavy-duty-flat-cars