@Bill Swatos - Here is the Marx 999 locomotive. Marx made only 3 die cast locomotives, the 999 was the first and most authors comment that it was likely created to work with the 3/16" scale cars. It is a 2-4-2 designed in 1941 and offered at least until 1959. The other 2 locomotives are the 333 which came along in 1949 and was the most detailed engine Marx produced, it was a Pacific 4-6-2 with a highly detailed casting for the boiler and came equipped with the ONLY die cast tender Marx produced (I have one and its so heavy you could use it as a door stop!!). The final engine was the 666 initially offered in 1955 and while it started as a die cast it was later converted to a plastic boiler and re-numbered 1666.
The picture below, shows the 999 with some of Marx's much later plastic boxcars. These were not really any scale and are short when compared with the Lionel 6464 series (8 1/2 " long vice nearly 10" for the Lionel).
Best wishes and thanks for your comment Bill
Don