Let us study for a moment the construction of the different types of railroad bridge. For the tiny creeks--the little things that are mad torrents in spring, and run stark-dry in midsummer--where they cannot be poured through a pipe or a concrete moulded culvert, the simplest of bridge forms will suffice. And the simplest of bridge forms consists of two wooden beams laid from abutment to abutment and holding the ties and rails of the track-structure. As the first development of that simplest idea comes the substitution of steel for wood, giving, as we have already seen, protection against fire and a far greater strength. The steel beam has greater strength than a wooden beam of the same outside dimension and yet in its design it effects for itself a great saving of material, by cutting out superfluous parts and becoming the structural standard of to-day, the I beam. When the I beam becomes too large to be made in a single pouring or a single rolling, it may be constructed of steel plates and angles firmly riveted together, and thus still remains the possibility of the simplest form of bridge. That single span may be further increased, or the bridge developed into a succession of increased spans by the substitution of the lattice-work girder, effecting further saving in weight without material loss of strength for the solid-plate girder. The track may be laid atop of such girders or--to save clearance in overhead crossing--swung between them at their bases. The limit in this form of bridge is generally in a 65-foot or a 100-foot span. It is not practical to build the girders up outside of a shop; and the 65-foot length represents the two flat-cars that must be used to transport any one of them to the bridge location. Some railroads have used three cars for the hauling of a single girder, and so increased these spans to 100 feet; but as a rule, over 65 feet, and the truss, the most common form of railroad bridge in this country, comes into use.