http://books.google.com/books?id=RdnZiHvabiMC&pg=PA112 (1918): lists on p. 36 (partial; also shows the New England Steamship Company as Class I), p. 112 (full), p. 151 (relinquished)

http://books.google.com/books?id=TKcTAAAAIAAJ&pg=PA1181 (1919): list on p. 1181 of railroads that were bumped up in 1919

http://books.google.com/books?id=QxYeAAAAMAAJ&pg=PA231 (1920): list on p. 231 (full, but also includes some "large terminal companies")

http://books.google.com/books?id=4kY3AAAAIAAJ&pgis=1 (1920 AAR): couldn't get a full list, but some information was used

http://www.nber.org/chapters/c4765.pdf : full lists on p. 52 (1928), p. 55 (1936), p. 58 (1940) - groupings are from this, and the group name is not listed separately in 1918 and 1920

http://books.google.com/books?id=hT5oAAAAIAAJ&pgis=1 (1951; includes history)

http://books.google.com/books?id=RSZPAAAAMAAJ&pgis=1 (1956; includes history)

http://cba.unomaha.edu/faculty/mohara/web/Koeppe_Competitive_RR_97.pdf : full list from 1970-1996 on p. 73