The United States Mail Operating Box Car No. 3428 was produced in 1959 and 1960 on Type IIB and Type III bodies. Painted RED, WHITE, and BLUE, with BLACK heat-stamped lettering on WHITE and WHITE heat-stamped lettering on the RED and BLUE painted portions of this 6464 Box Car derivative. The Multi-block doors were painted in matching colors. The car has a plunger mechanism that would open the door and the mail carrier would "pop out" holding a mail bag with the use of a UCS or No. 6019 track uncoupling section.

Enclosed with each car was Instruction Sheet No. 3428-23 that was dated 8-59. This sheet was printed with BLUE ink on WHITE paper. The front page of this sheet is shown below.

The mail bag was attached to the mailman with a magnet. The force of the plunger mechanism when activated would throw the mail bag clear of the box car. The color of the mail bag would be either BLUE or GRAY and this color would usually be opposite of the color of the mailman. This car came with early AAR trucks that were equipped with disc couplers.

This operating car has three variations:

Variation A: Type IIB body, GRAY man with a GRAY mail bag.

Variation B: Type IIB body, BLUE man with a BLUE bag.

Variation C: Type III body, BLUE man with GRAY bag.