To summarize then, electric working of a trunk line results in increased economy over steam locomotives by concentration of the power and especially by the use of water power where possible. Thus economy is secured to the greatest extent by a complete electrical service and not by a mixed service of electric and steam locomotives. Electrification gives an increase in capacity both in the haulage by a locomotive, an electric locomotive being capable of more work than a steam locomotive, and in schedule and rate of speed, as motor car trains and electric terminal facilities make possible augmented traffic, and an increased use of dead parts of the system such as track and roadbed. There is a great gain in time of acceleration and for stopping, and for the Boston terminal it was estimated that with electricity 50 per cent, more traffic could be handled, as the headway could be reduced from three to two minutes. The modern tendency of electrification deals either with special conditions or where the traffic is comparatively dense. From such a beginning it is inevitable that electric working should be extended and that is the tendency in all modern installations, as for example, at the New York terminal of the New York Central and Hudson River Railroad where the electric zone, first installed within little more than station limits, is gradually being extended. As examples of density of traffic suitable for electrification, yet at the same time possessing problems of their own, are the great terminals such as the Grand Central Station of the New York Central and Hudson River Railroad in New York City, the new Pennsylvania Station in the same city, and that of the Illinois Central Station in the city of Chicago. Not only is there density here but the varied character of the service rendered, such as express, local, suburban, and freight, involves the prompt and efficient handling of trains and cars. Now, with suburban trains made up of motor cars, a certain number of locomotives otherwise employed are released; for these cars can be operated or shifted by their own power. Such terminal stations are often combined with tunnel sections, as in the case of the great Pennsylvania terminal, where the tunnel begins at Bergen, New Jersey, and extends under the Hudson River, beneath Manhattan Island and under the East River to Long Island City. It is here that electric working is essential for the comfort of passengers as well as for efficient operation. But there are tunnel sections not connected with such vast terminals, as in the case of the St. Clair tunnel under the Detroit River.