I don't know what specific prototype these models might be based on (if any), but 50' ACF flat cars of this general type date back to at least the 1920s.
These models were also sold in two-packs with a long pipe load -
Additionally, they were also included as part of the BNSF Fuselage set -
Boeing receives their fuselages and other assemblies from Spirit Aerosystems in Wichita, Kansas by rail. Fuselages are regularly in transit between Wichita and Renton to keep up Boeing's production rate of more than forty 737's per month. This 2-pack features two BNSF flat cars, one plastic-injection fuselage, one resin tool box and supporting brackets. Fuselages come painted. Decals provided for fuselage details.