In 1950, Marx introduced the largest locomotive it ever made, the Marx 21 Santa Fe diesel. Marx made both powered and unpowered versions, and they were dressed up in the same warbonnet scheme as Lionel’s iconic F3 diesels, but unlike Lionel’s effort, they were nearly 1:48 scale (proper for O gauge) and made of metal.
Marx only made them for two years.
At some point, Marx planned a series of large tin lithographed freight cars that would match the scale of the #21 diesel, but never introduced them. As a temporary measure, they put their 1:64 scale freights on larger trucks so they would match the height of the #21s, but the cars were still shorter than one would expect.
Marx never released its 1:48 tin cars due to metal shortages caused by the Korean war. We would have expected this product line to survive until 1961 or so, but since Marx didn’t know for certain it would have a steady supply of metal to use, it accelerated its transition to plastic. Marx continued to produce its smaller, 6-inch tin trains, but its larger trains transitioned to plastics, which had the side effect of permitting a greater level of detail.
As part of the transition, Marx developed a line of plastic diesel locomotives to replace the #21. As a result, the #21 was discontinued after 1951.