This is an adaptor for placing the containers with round mounting sockets on a container car made for square container mountings or one made for truck trailers
This will mount N scale containers with round connection plugs.
example of cars that can be converted to carrying containers with round mounting sockets:
Fleischmann nr 845315
Fleischmann nr 845333
Fleischmann nr 845334
Fleischmann nr 845358
And plenty more