see [1] for a plot of pass-mi by mode for 1960+ (but it's not restricted to intercity travel). See "Transportation in America" for intercity travel figures going back to 1939.
see traffic flow (from MIT) See equation (2) near the end of this article. While this article is for the flow of vehicles on a highway it could also apply to the flow of trains, etc. on a railroad.
Data points in the graph come from many sources listed in the pageWikimedia: RailUSAvsUSSR.svg. A single source book "International historical statistics" (various editions), vols. "The Americas" and "Europe" also has this data (see Ch. on "Transport" in each of the 2 vols.)
worldcat.org
See the Soviet serial "Ежегодник книги СССР" (Yearbook of books published, USSR) published in Moscow by "Бсесоюзной книжной палаты = "National book publishers" ISSN0201-6354. For example, the yearbook for 1956 (vol. 2 for the 2nd half of 1956) shows books 20937-20789 for railway transport. The numbering starts with 1 in vol.1, 1956, and ends in vol.2 with 27738, the number of titles of books on all subjects published in the calendar year of 1956 in the [USSR].
