Just for the record... Extensive research with the New York Central System Historical Society identified the color pretty positively.
Trying to define a color is extremely difficult. Paint formulas don't work, because pigments go out of production (e.g. because lead and chromates became illegal). Even Pantone colors are themselves derived from pigments, hundreds of Pantone colors will soon die because Pantone is cutting back the number of pigments in their system.
There are systems which define colors in an absolute way: wavelengths of light, reflectivity, etc. One of these is
the Munsell system. It can use decimal points to describe any color. But Munsell sold chip books of the "even numbers". It was pretty much the Pantone of the 1930s-50s. Quite often, stylists would simply pick an Munsell chip out of the book, same as you might pick a Pantone color today.
After a few rounds of trying to match up colors, it looked like NYCSHS's original NYC drift card was
very close to "Munsell 2.5 BG 5/8", which is one of the chips in the book. That seemed to me like too much of a coincidence. I felt the small difference could be accounted for by the sample being 50 years old.
The color is definitely not RAL 6027.
I ordered a fresh sheet of 2.5 BG 5/8 from Munsell, and sent it to Awlgrip, a top maker of yacht paint. They created a formula for Central Green, their H4682.
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(If you're wondering how 2.5 BG is "even", it's because Munsell uses 10 major hues around the color wheel with 10 divisions of each. But to save money they only sell 40 hues in their color books.)