Downy young King Rails can be mistaken for Black Rails, but have dark (not red eyes), a white bill, and lack spotting on the back and barring on the flanks. The smaller Virginia Rail has a redder bill and much more contrast between the gray cheeks and the cinnamon underparts. The King Rail is very similar as an adult to the Clapper Rail, but is more frequently found in freshwater marshes. Adults are easily separated from East Coast Clapper Rails by their rich reddish brown head, neck and underparts without contrast between the cheeks and body plumage. The reddish upperwings separate King Rails from all races of Clapper Rail.
Length and wingspan from: Robbins, C.S., Bruun, B., Zim, H.S., (1966). Birds of North America. New York: Western Publishing Company, Inc.