What cut of beef is "plat nerveux"? The cut is 'plat nerveux' and was purchased at a Parisian (French) market. It is beef, long-ish, round (like a sausage), and boneless. The meat is deeper red than most beef cuts I am used to. Deep red, but not liver or other organ color.

This cut is not even listed on https://tasteofsavoie.com/2018/10/23/french-meat-cuts/
Different countries can have very different names for their cuts of meat, and in some cases, there isn't an obvious/direct equivalent from a French cut to an American or English cut. If you look at a French butcher's diagram, you'll see the lines and cuts don't correspond directly to a US diagram.

I believe that what you saw labeled as "plat nerveux" is likely "gite de noix" which in the US is "eye of round roast". "Nerveux de gite" is a braised beef dish that is kind of like an American pot roast. 

Eye of round can have a shape similar to a tenderloin, but is a tougher, less expensive cut of meat.