in a case where a double-track, steam-railway bridge had to carry in addition a double-track electric-railway, wagon-ways, and footwalks. The choice lay between a double-deck structure with the steam railways below, a combined double-track electric-railway and wagon-way above between trusses, and footwalks outside of the wagon-way; and a single-deck structure with the double-track steam-railway between trusses as before, a combined single-track electric-railway and wagon-way on cantilever brackets outside of each truss, and a footwalk outside of each wagon-way. For reasons unnecessary to state, the gauntleted tracks between trusses for the electric railway were barred. The comparison was hardly a fair one, because the single-deck layout provided facilities superior to those afforded by the other layout in relation to rapid transit, there being two roadways of twenty-two feet each instead of one roadway of thirty feet-besides avoiding a climb of some twenty-five feet. While the cost of the main spans was greater for the single-deck structure than for the double-deck one, on the other hand there was a saving in the lengths and costs of the approaches. It is impracticable to make any general statement of comparative costs or advantages of these two types; and each case as it arises, will have to be worked out by itself as a special economic problem. Moreover, one layout or the other, irrespective of cost, will be preferable from the point of view of service or accommodation, hence this feature will have to be given serious consideration.

As a rule, bridges for carrying both railway and highway traffic are located in or near large cities, although an occasional structure of this kind is found in country districts. The principal advantage of this type of bridge is the saving in first cost, and its principal disadvantage is a reluctance to cross over it on the part of timid drivers, whose horses may be frightened by the trains. The saving in first cost of a combined railway and highway bridge, as compared with two separate bridges for railway and highway traffic, is considerable, because the piers for the combined bridge are but little, if any, more expensive than those for the railway bridge, and because the extra metal for the super-structure of the former in comparison with that of the latter is very much less in weight than the metal required for a separate highway bridge. The prejudice against combined bridges on account of danger is almost wholly unfounded, for horses soon become accustomed to railway trains, and, when screens are employed to hide the latter, but little trouble is experienced on account of frightened animals. These screens may be made either slatted or solid, the former offering less resistance to the wind, and the latter being the cheaper. Moreover, automobiles have almost entirely displaced horses in highway traffic.

The advent of the electric railway has somewhat complicated the question of designing combined bridges, for now it is often necessary to accommodate all kinds of traffic on the same structure.

Combined bridges may be divided into the following classes:

1. Structures having a single deck for all kinds of traffic, the railway