The Pipe Flat Car No. 6121 was available in 1956 and 1957, and was created on a painted stamped steel chassis with heat-stamped lettering. Early cars would have bar-end trucks with magnetic or tabbed couplers, while later editions have early AAR trucks equipped with disc couplers. Came with three to six pipes and these are easily interchangeable with the similar No. 6111 log car.

Those variations that were produced in 1956 have bar-end trucks and in 1957 Lionel put early AAR trucks on this car. This car is found in boxes that were marked 6121, 6121-25, 6121-60, and 6121-85.

There are a total of seven variations:

Variation A: DARK GRAY chassis with serif lettering. Came with six plastic pipes and early AAR trucks. Sometimes found in boxes that are marked 6121.

Variation B: CREAM chassis with BLACK sans-serif lettering. Came with three plastic pipes and early AAR trucks.

Variation C: YELLOW chassis with BLACK serif lettering. Came with bar-end trucks. This car is sometimes found in boxes that are marked 6121-25.

Variation D: BRIGHT RED chassis with WHITE sans-serif lettering. Came with three plastic pipes and early AAR trucks. Sometimes found in boxes that are marked 6121-85.

Variation E: FIRE ENGINE RED chassis with WHITE serif lettering. Came with three plastic pipes and bar-end trucks. Sometimes found in boxes that are marked 6121.

Variation F: BRIGHT RED chassis with WHITE serif lettering. Came with three plastic pipes and early AAR trucks. Sometimes found in boxes that are marked 6121-85.

Variation G: YELLOW chassis with BLACK serif lettering. Came with three plastic pipes and early AAR trucks. Sometimes found in boxes that are marked 6121-60.