Aren't they actually designated as 'Well Cars' ?
Definition of WELL CAR .........................:  a railroad flatcar having a depression or opening in the center of the  deck for handling oversize loads that would not on regular flat cars  come within overhead clearance limitations —called also well-hole car 
Definition of FLAT CAR .........................: a railroad freight car without permanent raised sides, ends, or covering 
Above info taken from :- 'Merriam-Webster' an Encyclopaedia Britannica Company.