Source: Data For 1920-1931: Ministry Of Transport, "Railway Statistics", Succesive Issues Of 1920-1931. Data For 1931-1939: Computed By NBER By Dividing Series (Net Ton-Miles) By Series (Loaded Wagon-Miles).
This NBER data series m03077 appears on the NBER website in Chapter 3 at http://www.nber.org/databases/macrohistory/contents/chapter03.html.
NBER Indicator: m03077
Release: NBER Macrohistory Database
National Bureau of Economic Research, Average Wagon Load for Great Britain [M03077GBM464NNBR], retrieved from FRED, Federal Reserve Bank of St. Louis https://research.stlouisfed.org/fred2/series/M03077GBM464NNBR/, July 6, 2015.