author classes it simply as a highway bridge, for it is seldom necessary to strengthen it materially, because of the electric-railway load, except in the floor and primary truss members. But if a structure has to carry either a single or double track electric railroad only, the author treats it as a railroad bridge.

Combined bridges may be divided into the following classes:

1.   Structures having a single deck for all kinds of traffic, the railway occupying the centre of the bridge, and the electric railway lying close to one truss.

2.   Structures having a single-track railway at the middle, a narrow footwalk on each side of same inside of the trusses, and cantilever brackets outside of the latter to carry wagonways and electric lines. This arrangement may be varied by running the electric cars over the main railway track, thus leaving the wings free for wagon traffic.

3.   Structures having a double-track railway inside of the trusses, with long cantilever brackets outside carrying wagons and electric lines next to the trusses, and pedestrians outside. This arrangement may be varied, as in Case 2, by carrying the electric trains on either one or both of the main railway tracks.

4. Structures having a double-track railway inside of the trusses, with short, cantilever brackets for wagon and electric-railway traffic outside, and either a single passageway overhead at the middle for pedestrians, or two passageways for same on overhead brackets outside of the trusses. As before, this arrangement may be modified by running the electric trains over the main railway tracks.

5.   Double-deck structures carrying railway trains on one deck and wagons, electric trains, and pedestrians on the other. The pedestrians may be accommodated either inside the trusses or, preferably, by exterior walks on cantilever brackets. The railway may be placed either above or below to suit the existing conditions, or there may be either a single-track or a double-track railway both above and below; with wagon-ways and pedestrian-ways outside of the trusses either above or below.

Class No. 1 is the cheapest possible kind of combined bridge,