In addition to these stations, each large manufacturing plant is apt to be a freight station of itself, with a private switch running to its shipping-rooms and storage sheds; and in even a moderate-sized American city there may be from 300 to 500 of these sidings in active daily use. So much for the general commodity freight. Then there are the special commodities. Coal, for instance, is a freight business of itself. It is not handled in the regular stations of the railroad, but in specially designed pockets and storage sheds, which may be located at from one or two to half a hundred different accessible points about the city. One begins to see, after a little while, why the railroads now seize with avidity each opportunity to gain lines through the hearts of our cities. Each line gained means some appreciable relief toward the taking up of a traffic burden that increases yearly. It is most probable that the freight terminals of the city will have to accommodate much more traffic than that which originates or terminates there. Important lines of other railroads may intersect at that point, and the handling of interchange freight is a busy function of the terminal scheme. It may be an important point for lake, river, or ocean traffic; and in such a case, the industries at docks and docking facilities of every sort form other busy functions. There will be coal or ore wharves, elevators, and car-floats to enter into the scheme. So you see the railroad's freight terminal in any large city is like the fingers of its extended hand. The long tendons reach into every productive centre, gathering and distributing at from a dozen to fifty points, aside from the private sidings. It is obvious that these must be caught together somewhere; and generally upon the outskirts of an important traffic city the railroad creates an interchange yard where this freight, incoming and outgoing--100 trains a day, perhaps--is gathered together and sorted with system and regularity, very much as the post office sorts the letters and the mail packages. To examine more closely this working of a modern freight terminal scheme, let us take a single plant of a single system. The great operation by which the Pennsylvania Railroad catches up and delivers its freight in the metropolitan district around New York is typical, and will illustrate.