Marx made a variey of large diesels in the postwar era. The first and only large tinplate diesel was the Marx rendition of an EMD (Electro Motive Division) FT,  the Santa Fe "AA". The #21 was introduced in 1950 to compete against Lionel's phenomenal success that was generated by their introduction of the Santa Fe 2333 diesels. The Marx #21 tinplate diesels are pictured below on the top. Just as an aside, a gorgeously decorated Rock Island Diesel (actually a model of an EMD FP7A) was offered to compete with Marx by a company by the name of Unique Arts. You can see them below. They are of comparable size and tin plate as well:                               

The large plastic diesel is most similar to an EMD E-7. The E-7 diesel era began in 1952 with the introduction of the #1095 Santa Fe "AA"'s with their "6-wheel" simulated trucks:

Though the photo shows an "ABA" combination the initial offering was the "AA". The "B" unit wasn't introduced until 1955. Note the second "A" unit and you see the left step is missing just to the left of the rear truck. This is a common defect as the steps were fragile. Reproduction steps are available which can be glued and look very original if done with care

The Marx plastic E-7's dominated the large plastic diesel production for many years (1952 through 1974). They were available in the following road names: Santa Fe, New York Central, Union Pacific, Rock Island, Allstate, Western Pacific, New Haven and Penn Central.

Most prolific in production are the Santa Fe's, New Haven's and the Rock Island's:

Marx sets came in various freight and passenger combinations but the most "prestigious" were the "ABA" combinations. These "ABA"'s were offered in Santa Fe, Rock Island, Western Pacific and Allstate only. The "B" unit was especially configured with a tab and slot coupler on either end which matched the "AA" coupler configurations. Any "B" unit will connect but the other "B" units came with a tab coupler on one end and a fork coupler on the other. The fork coupler had a slot so an "ABA" combination could be assembled.

The New Haven , New York Central and Santa Fe caboose offerings came with "stunning " tri-color decoration. Below is the New Haven on the left and NYC on the right and Santa Fe below:                

The triumvirate of spectacular Marx cabooses made an exquisite addition with the New Haven diesels in particular.  One variation of the Santa Fe has the color reversed. I have seen only 1.

A Santa Fe "AA"combination can be found for between $25 and $50 and the tricolor matching caboose for $10 or so. How can you beat fun with Marx trains at these prices!

Very best, Mike (lionelsuperotrack on eBay)

PS I would like to acknowledge John F. & Tasker B. for introducing me to Marx trains. Their continued willingness to answer my questions and share information is most appreciated. Their input has been invaluable and gracious in every instance.