The Lionel Hudson Type 4-6-4-Locomotive No. 646 boiler is derived from a modification of the No. 726 Berkshire locomotive it shares this same boiler with No. 2046 and No. 2056. It was available from 1954 until 1958. This engine was equipped with Magnetraction, a hinged boiler front, smoke, drive rod connecting rod valve gear, three position direction unit that is adjustable from the top of the boiler, and there is a whistle in the tender. It came with both rubber and heat-stamped numbering on the cab.

Early examples would have a die-cast trailing truck with the number rubber-stamped on the sides, while later versions would have a trailing truck with plastic side frames and a heat-stamped number on the cab.

There are three variations:

Variation A: The number designation is rubber-stamped in large SILVER numbers on the cab. This variation came with a metal trailing truck, and a No. 2046W tender that had bar-end trucks with one magnetic coupler. 1954 production.

Variation B: WHITE rubber-stamped small number on cab. and a No. 2046W tender that had bar-end trucks usually with one magnetic coupler.

Variation C: WHITE heat-stamped small number on cab. and a No. 2046W tender that had bar-end trucks usually with one tabbed magnetic coupler.