Here again there is further division. A railroad in its physical aspect consists of the track that things run upon and the things that run upon the track. The track, in the broad sense in which we are now considering it, consists of far more than two steel rails set upon wooden ties or sleepers which, in turn, are set in a graded roadway. It means bridges, tunnels, switches, signals, terminal and intermediate stations, buildings, passenger and freight-houses, engine-houses, shops, and all the rest of it. And upon track, in this and every other sense the things that run, are to be translated as locomotives, of a variety of forms, and cars, of an infinite number. And because cars are, as a rule, quite helpless without locomotives to push or to pull them here and there, let us begin with the locomotive. For the moment, we are going to pass by the steam locomotive, and the large possibilities of its development far beyond the present point, and come direct to the form of tractive power which has at least the most popular appeal to the modern imagination--electricity. The use of electricity as a motive-power upon this country's so-called standard railroads (the electrical engineers like to call these heavy traction railroads) is no novelty. It began nearly thirty years ago when the Baltimore and Ohio railroad completed the electrification of its then new tunnels under the City of Baltimore. The move was made primarily to remove offensive smoke conditions, particularly in the main tunnel connecting Mount Royal and Camden stations, nearly two miles apart. In fact to-day trains going from Mount Royal toward Camden, a steady down-grade, are operated without the trouble of attaching electric locomotives to them; it is an easy gravity run for the two miles. For the up-trip the electric locomotive is attached at Camden Station in front of the through steam locomotive of the train and finally detached about two miles east of Mount Royal, by the simple process of running ahead and upon a facing-point switch--an adaptation of the old-time "flying switch."