But the prudent Americans have availed themselves of other sources of economy by adopting a mode of construction adapted to the expected traffic. Formed to carry a limited commerce the railways are generally single lines, sidings being provided at convenient situations. Collision is impossible, for the first train that arrives at a siding must enter it and remain there until the following train arrives. This arrangement would be attended with inconvenience with a crowded traffic like that of many lines on the English railways, but even on the principal American lines the trains seldom pass in each direction more than twice a day, and their time and place of meeting is perfectly regulated. In the structure of the roads, also, principles have been adopted which have been attended with great economy compared with the English lines. The engineers, for example, do not impose on themselves the difficult and expensive condition of excluding all curves but those of large radius, and all gradients exceeding a certain small limit of steepness. Curves of 500 feet radius, and even less, are frequent, and acclivities rising at the rate of 1 foot in 100 are considered a moderate ascent, while there are not less than 50 lines laid down with gradients varying from 1 in 100 to 1 in 75, nevertheless these lines are worked with facility by locomotives, without the expedient, even, of assistant or stationary engines. The consequences of this have been to reduce in an immense proportion the cost of earthwork, bridges, and viaducts, even in parts of the country where the character of the surface is least favorable. But the chief source of economy has arisen from the structure of the line itself. In many cases where the traffic is lightest the rails consist of flat bars of iron, 2-1/2 inches broad and 6-10ths of an inch thick, nailed and spiked to planks of timber laid longitudinally on the road in parallel lines, so as to form what are called continuous bearings. Some of the most profitable American railways, and those of which the maintenance has proved least expensive, have been constructed in this manner. The road structure, however, varies according to the traffic. Rails are sometimes laid weighing only from 25lb. to 30lb. per yard. In some cases of great traffic they are supported on transverse sleepers of wood like the European railways, but in consequence of the comparative cheapness of wood and the high price of iron, the strength necessary for the road is mostly obtained by reducing the distance between the sleepers so as to supersede the necessity of giving greater weight to the rails.