The transportation and communications industry which “sells change in location” (Capital, vol. 2, p. 52) is directly productive of value because “economically considered, the spatial condition, the bringing of the product to market, belongs to the production process itself. The product is really finished only when it is on the market” (Grundrisse, pp.533-534). However, the means of transportation and communication, because they are made up almost entirely of fixed capital, have their own peculiar laws of realization (Grundrisse, p. 523)—laws which stem from the fact that transportation is simultaneously produced and consumed at the moment of its use.
From David Harvey‘s essay “The Geography of Capitalist Accumulation: A Reconstruction of the Marxian Theory” (1975).